LePhuronn :
At the end of the day it just comes down to your frame rate. The monster graphics set-ups you're talking about will handle pretty much anything at any resolution when paired with a good processor.
Otherwise it's a balance between CPU, GPU and playing resolution to get the best frame rates as you can. We can only advise on that when we know exactly what you're going to run.
Mostly Crysis and Crysis Warhead on very high/enthusiast on 1024x768 0x AA and motion blur disabled. Most of today's top-end graphics cards can run any game on maximum detail, it's just AA and extremely high resolutions that kills performance. High resolution is not the difference between night and day like many people say it is. It can make the difference between an enjoyable gaming experience, and an extremely slow, unplayable one.
Apparently, if you can max Crysis, you will be able to max Crysis 2 due to the many optimizations and flawless coding of the CryEngine 3 which the CryEngine 2 did not have.
A single HD 5870 or GTX 480 are overkill for those games on that low resolution, as I've been told. It's 1920x1080 and beyond, combined with anti-aliasing which really gives graphics cards a run for their money.
The only GPU setups which can fully max Crysis (playably) are Two superclocked HD 5970's in xfire, and 3-4 way GTX 480's in SLI.