The latest Linux titles should run fine on those computers, as I recently purchased a Dell Opitplex 740 for $29 plus shipping (no OS, but a Vista Business COA was attached). Though it has an AMD CPU rather than Intel, that I upgraded twice, one that I had on hand & another that I found on eBay for $10, and also upgraded the installed 2GB of PC2-5300 RAM to two sets of 4GB RAM kits for 8GB total, plus added a 1GB GDDR5 AMD Radeon 7570 (Dell rebranded), which was the GPU my XPS 8700 shipped with laying around serving no other purpose.
Plus added a faster HDD that was already here, and I was in business. It ran Linux Mint 17 LTS 64 bit w/out any changes, but was a bit sluggish, so grabbed the new RAM sets while still available. Except for the $10 for the more powerful CPU, the rest I had. Oh, I forgot to add, also had a nice, like new ASUS DVD/CD RW optical drive (a Newegg 20x Customer Award winner), that I swapped out the old unit with. It likely runs better than the day it was first removed from it's box.
One reason why if you begin to work on computers & find yourself with parts left over, don't trash them.
Most of these computers are XP era computers and I would have thought it possible to still find drivers so that we could use Windows 7 basic, but this thought process has proven me wrong.
Yeah me too, have had a couple that I thought this could be done on, but drivers are an issue with many older models, with the chipset drivers being critical to other functions. Mint 13 is the latest LTS release for these, supported until April 2017. There is also a less advertised Ubuntu 12.04 non-PAE edition, that Canonical didn't originally want released, and it's still not on their official site, but it's available. The top link is the one you need, the bottom is beta.
Source: Post #4 of this page. Note that the first two posters was trying to talk the OP into watered down versions of Ubuntu & the OP solved his/her own Topic. Rather than a straight to the point answer, those posters was offering the OP a solution that would have involved a lot of work, and still not a fully loaded Ubuntu, what was asked for. Those long term members likely knew of the non-PAE 12.04, falling into Canonical's vision of 12.04 being PAE for everyone.
Fortunately, the devs of Linux Mint were more sensible & offered one last LTS release, and the last one altogether, that would run on non-PAE computers, of all of their versions, though it's best to have at least 1GB for MATE, and 2GB or higher for Cinnamon & KDE. The 64 bit versions may require a bit more, as I stated, 2GB wasn't quite enough for everyday use.
Though the Optiplex 740 runs Windows 7 Ultimate, Vista Business & Linux Mint 17 perfectly. The RAM & CPU increase paid off, with WEI scores of 6.9 on RAM, and both video scores, those GPU scores were the same as when the card was in the XPS 8700.
My only issue with the machine, the motherboard is a bit odd, it's powered by a NVIDIA chipset & there's been no work on it since 2010. What that means is though it runs SATA HDD's, at the SATA 2 standard, but is officially recognized as IDE, meaning that an SSD install was out of the picture. This, I didn't notice until I installed one to secure erase, and it wouldn't, it was reporting a IDE controller.
It would seem that later versions of Windows & Linux would offer their own SATA controllers for the board, to be in full compliance with SATA specs. In essence, this is like having a native SATA PC running in IDE mode, many ran XP on some earlier model computers with SATA (2006 through 2011) in this BIOS mode, and there was no speed penalty on HDD's. Did it myself on an HP that I finally tore apart.
However, this mode isn't ideal for SSD's & would lead to a much lessened lifespan. So I didn't install one as planned. Maybe at some point, I can find a motherboard that's true SATA that'll fit perfectly at the right price. For now, it's OK as is.
Out of all of those Optiplex PC's you have, the ones with the Intel Core 2's are the best. If possible, swap the larger RAM sticks from the Celeron & Pentium models into the Core 2's, as the Celeron is a waste anyway, it's for Intel cheapskates who doesn't mind a rock bottom CPU. Runs like snail mail compared to email. There's no reason for those Core 2's to be running anything less than 2GB RAM, though looking at the count, one may have to stick with 1GB.
The Pentium was once a great CPU, but in recent years, even their dual core models, doesn't come close to competing with an i3, their mid-line CPU, and the most affordable of the 'i' series. I have a 4 year old i3 that would smoke a production Pentium (or Celeron) of today.
Those Core 2's holds a lot of promise, and would make fantastic Linux computers, or even Workstations. Most of the Optiplex series were geared towards business anyway, so they're built good. I would have bought one of those over the one that I have, but none had the hard drives or caddies, and these were 'as is' only sales, whereas mine has a 30 day warranty. I'm not purchasing 'as is' computers from eBay sellers.
The rest, while they're not a total lost cause, will have trouble running a full version of Linux, and the one with the oldest CPU, the Pentium 2.8GHz of 2002 origin, won't make the cut to run the latest titles. I had one of those a couple of years back.
1)DEL OPTIPLEX 745- Intel Core 2 6600 2.4GHZ, 512Mb ram, 64 BIT OS
2)DEL OPTIPLEX 745- Intel Core 2 6600 2.4 GHZ, 1Gb ram, 64 BIT OS
3)DEL OPTIPLEX 745- Intel Core 2 6600 2.4GHZ, 512Mb ram, 64 BIT OS
And these should, with their specs, should have 4GB (or more) RAM out of the box. I have a hunch that someone swapped some components around on these. Same with this one.
7)DEL OPTIPLEX 745C- Intel Core 2 6700 2.66GHZ, 2Gb ram, 64 BIT OS
I see another Pentium with 3GB RAM installed, what a waste of good memory. If possible, and if in two sticks, try to beef up those Core 2's a little, and place one of those 512MB sticks in that one. Or swap the sticks with the Pentium D, that should be another good CPU, the prime of the Pentium days. That other Pentium (2002 model above) & Celeron is the least of the performers. Any good components that can be swapped from those into the ones with the better CPU's, should be. This is to include optical drives, quieter fans, and if you are familiar with backup software, the hard drives too. Though I'd imagine that the one with the 2002 CPU has an IDE HDD. Dell was still using these in 2004 on some models. It likely has plain old DDR RAM. That one, you can play around with, but don't expect to get more than $25, if that for it. On recycle days, there's nicer ones to be found on the curb.
So it appears as though you have 5 PC's there that are capable of running the full versions of the latest & greatest Linux titles & worth something. The one with the Celeron is 100% Xfce only, and it'll fight it way through running that. No matter how much RAM is installed. The oldest Pentium, though can run Mint 13 only, will give the Celeron a run for the money.
Dell must do some funky model numbering, mine's a 740 (lower model number), but newer components. I've noticed this in the past with their Latitude line of notebooks, some of the lower numbers of a series (such as 'C' or D') are newer than the higher ones & wonder if this is a marketing ploy.