My HP dc5800 took a upgrade from a Core 2 Duo E8400 (3.0GHz) to a E8600 (3.33GHz), though had to use a special HP microcode (maybe supplied by Intel) for it to work, and had to perform the microcode update prior to installing the upgraded CPU.
Did consider a C2Q 9550/9650, though already have an Optiplex 780 with that one, plus the PSU of the PC would have more to handle, so went with the E8600. All was running fine until I performed a Windows 10 upgrade, and once it booted to desktop, it was over, just like that. Tried several times to revive the BIOS with a Win98 rescue USB drive (reloading the BIOS), it came up once, and never did again.
So IMO, Louis provided a good link to what appears to be a mouth watering deal for me.......and I don't need anymore computers of any type, though if weren't saving for a much larger upgrade, would pull the trigger after a couple of questions. The market is flooded with these former business/corporate systems, one of the best things I like about these are drive security, if one has Windows 7 Ultimate, 8.1 or 10 Pro, then Bitlocker is there for drive encryption......for every one installed, plus any USB drives attached. There's no better encryption than hardware level, even if the standard is one version behind.
Should you get one of those from one of the better rated sellers (the 2nd or 4th link on the page), these looks to be good PC's from reputable sellers, I never purchase anymore from sellers with less than 95% Feedback, except in the rare instance where I need a component & there's no other choices. Then will ask questions before purchase. Always ask questions if there's anything of concern before pulling the trigger.
As a last note, it's best to go with a refurbished computer that shipped with a minimum of Windows 7, Vista/XP computers are less likely to be in good condition, and some sellers will cover up (which is against the rules of being a Microsoft Authorised Refurbisher), if refurbished, both the original COA & the new one are supposed to be affixed to the computer. I know, have two of these systems, one purchased from an eBay seller & the one with the C2Q 9650, running fantastic. another from Newegg, the HP dc5800 that's kicked the bucket after what looked to be a BIOS corruption caused by a Windows 10 upgrade. Prior to that, Windows 7 was running fine, and even ran some of the W10 Previews on it w/out issue. The version that done the damage was the Anniversary Edition.
So on some of these, it may be best to shy away from Windows 10, even if there's an easy upgrade path, and there's still an open one, under the disguise of the need for assistive technologies (the site says 'no proof required). Just an open back door for those who said 'No' to change their mind that I've used with success on several occasions for myself & others.
Just don't do it with an old HP computer of any type, their 'softpaq' updates tends to be funky at times, and for the best experience, use IE for the download. Though don't do anything w/out first imaging the computer to an external, and if there's an opportunity, create any OS recovery disc set offered, and whatever, don't touch the recovery partition if one is present, it can be used with a partition tool to be made active in an emergency to reload the OEM OS back onto the computer. Many gets rid of this w/out thinking of the future, most of the refurbished systems I've purchased shipped with a Recovery partition & are still on the computers.
Good Luck in your hunt, though I feel that Louis pointed out an excellent deal, if purchased from the 2nd or 4th listed seller (as it stands now).