I am interested in setting up a gaming rig, and wanted to know if 4x GTX590 SLI cards are possible, if not, would I be correct to assume that only 2x GTX590 are possible which is not as good as 4x GTX580 cards?
Four GTX-590 cards all in SLI are not possible, because each GTX-590 has two discrete GPUs on each PCB. Even NVIDIA's own website states that two GTX-590's will technically perform as a Quad-SLI configuration.
You would be correct in assuming that four discrete GTX-580s all SLI'd together would outperform two discrete GTX-590s. That being said, both setups are technially Quad SLI. The reason the four 580's would outperform the two 590's can be found in the NVIDIA specification sheets (here is the GTX 580 versus the GTX 590). If we compare the two:
So, assuming performance scales linearly, you can see that doubling the GTX 590's specs is infact less then the GTX 580. Since both are a multi-GPU setup, you can also assume that performance losses through SLI scale the same in both setups.
Do note that with the quad GTX 580 setup, you would have massive power and cooling requirements. You would need to include at least two power supplies in your build.
Other reasons why a 4 card GTX 590 SLI setup isn't (currently) possible:
So when a benchmark like 3DMark seems to hint at a 4 card configuration (as in here), it is in fact incorrect. It counting cards from the perspective of the graphics driver that sees each GPU as a single card. With dual-core cards setup, this math is no longer valid.
You should go for 4x GTX580 as they overclock better according to this forum post.
This is backed up by the 3DMark Hall of Fame.
With that out of the way, proceed to The Real Deal, for a hands-on experience of such a system.