And to continue a little...(I drank some coffee earlier, so I'm still wide awake, lol)...
Adding tons of graphics processing alone doesn't necessarily equate to uber performance. The multi-GPU scenario has to be used along with a powerful CPU and fast system memory as well. Remember, "computing" requires an entire system and if you have one component that is particularly weak compared to the others you'll get poor performance.
The two main scenarios that will cause SLI or Crossfire to not render double frame rates are:
1. CPU bottlenecking - this occurs when the program you are running (typically a game), is "heavy" on CPU processing requirements. The effect is that your multiple GPU's are loafing waiting for the CPU to do its processing. If you have the current, highest end CPU, then there is nothing you can do at that point really...adding another GPU to the mix won't change a thing.
2. The code (the program itself and the way it is written) doesn't make efficient use of multiple GPU's or CPU cores. Some folks incorrectly assume that simply having more GPU's, or even more cores in the CPU automatically means improved performance and that's not the case. The software HAS to be written so that it will use the extra cores and/or efficiently make use of multiple GPU's. It's called "multi-threading". Depending on the purpose of the software, it can be difficult, if not impossible to multi-thread it. Operating Systems typically aren't multi-threaded, although it plays a part in directing multi-threaded operations with software that does.
Some software that is multi-threaded can only use 2 cores, so even if you had a 4-core CPU, it's only going to use 2 of them. Games are notorious for being single threaded, or maybe dual threaded at most today (although will change over time) and is the primary reason hardcore gamers today will pick say, an I5 or I7 CPU over one of the 6 or 8-core Haswells.
An I7-4790K has 4 cores and a clock speed of 4.0GHz native. This CPU is currently considered the "king" for building a gaming rig. I personally have an I7-5820K, which is 6-cores, but only runs at 3.3 GHz native. In a gaming environment, that 4-core I7-4790K beats my 6-core because games use at most 2-cores and the cores on the 4790K have a 0.7GHz clock speed advantage. The 4-core wins in that scenario....strictly speaking in terms of absolute frame rate in games...but realistically, that 5820K paired with a good GPU will play every game that the 4790K will at the same settings well enough that the differences are negligible. This is where all the hoopla in forums and "bragging rights" come into play and where many "fanboy" arguments occur about who's system is most badass at playing <insert your favorite game here>. It's sort of the modern world rendition of who's hotrod car is the fastest and most powerful.
But....step outside games and start running virtual machines, or Photoshop, or some other software that will use all the CPU cores and I'll leave them in the dust.
Additionally, Intel uses "Hyper Threading" technology. In essence, it allows two threads to be assigned to a single core. The effect is that computer system "thinks" it has double the amount of cores it physically has. So, the I7-4790K, with hyper threading can effectively "see" 8-cores. My I7-5820K "sees" 12-cores.
So, if I were to set up a virtual machine (basically a computer running within a computer) I have the option to assign any number of cores to that virtual machine. I could assign 6-cores to my local host (the main PC and native OS) and 6-cores to the VM. That 4790K can only do 4 and 4 at most. Now run, say Photoshop on the native machine and the VM simultaneously and guess what? My 5820K will smoke the 4790K!
For the average user needing basic day-today computing and gaming, the quad core I5 CPU is probably the best bang for the buck (but does not support hyper threading). The 6 and 8 core Haswell family of processors are a fair bit more expensive and unless you have a need for 6+ cores, you;re just throwing away money. The I7's do support hyper threading, but cost more than the I5's and unless you really need hyper threading are really a waste of money too...unless you just have to have the fastest gaming CPU around.
The 5390X 8-core I7 is a touch over $1,000 just for the CPU (and it only runs at 3.0GHz clock speed). After that, you have to step up to the enterprise class CPUs (the vaunted Xeon processors) and the upper level Xeons cost several thousand dollars. These really are specialized CPU's intended for use in servers or very high end workstations.
Beyond all those numbers, the good old hardcore gamers typically get into "over clocking". Basically they tweak the various clock multipliers and voltage settings to run the CPU, memory and GPU's at higher clock speeds than stock in order to get more processing power from them all in the name of higher frame rates in their games (again, this is the modern equivalent of hotrodding your car). This is specifically why if you were to go to say, newegg and searched for CPU heatsinks you'll find this vast array incredible solutions...I mean some of these aftermarket coolers are simply HUGE! They are designed to absorb and dissipate all that heat when you overclock (because overclocking will cause your CPU to generate a lot more heat). It's spawned the liquid cooling industry as well.
I used to water cool back before water cooling became all the rage :-) In the late 90's when the first "Covington" Celerons came out, I had one of the 300MHz versions. I discovered that if I sawed off the square knob on the top of a threaded 4" PVC pipe cap, it fit perfectly over the CPU slug (IHS). Some silicone secured it to the CPU. A couple holes tapped in the side, some tubing and fittings, an aquarium pump and a bucket of water and I was keeping my little Celeron jacked up to 464MHz plenty cool. Then my kids started walking and well...the thought of a kicked over bucket wasn't good. Besides, by then I moved on up to a "real" CPU and never really found the need for overclocking anymore (and I have never liquid cooled a CPU since). I almost dabbled in "submersed motherboards". The idea being to literally put your entire motherboard with CPU, memory, etc into a case of de-ionized water or oil. I know several folks who did it, successfully, but it was a maintenance nightmare. Constant leaks, etc. I just never found the need for such extremes anymore.
Overclocking in the world of gamers has become mainstream. Many CPU or GPU reviews you see online almost always contain data on "OC'd" set ups. The reality is that you seriously chance damaging your components, or shortening their life span and the gains are not worth the risk...but kids these days just have to have the fastest and the baddest of the bad ;-P