Hi , I don't know what you have for system spec's but I would get a GTX 580 with 3 gb mem for reasonable price and then add another later on for sli. The 3 gb will help with the resolutions and you will have an easier time with higher settings.The 2 580's with 3gb ea will stomp the 590 , the 590 comes clocked at 630 mhz and the 580 3 gb comes clocked at 772 mhz (or 783). I have 2 Evga GTX 580 FTW Hydro Copper 2 3 gb cards in sli and I play any game with all settings maxed out and the game detail looks great and I don't know how it could be any better.
Ofcourse 580sli will smoke the 590. But how long until you have the budget to buy another 580 and can you deal with the level of performance with just one 580 for now??
If your display res is around 1920x1200 only one 580 will max every game out but you will not get the smoothest framerate, Metro 2033 maxed will only give you framerates in the low 30s while one 590 now will give you mid 50s. For one $100 more the 590 sounds like a better deal. If you have higher display res, 3d, or multiple monitors one 580 would be unplayable on max settings on some of these GPU hungry games(3d alone cuts your FPS in half, 3 monitors in a 1/3), plus one 580 does not have connections for 3 monitors by itself, 590 does.
If you look at the benchmarks the 590 is the better deal for the money between the two and i would go with the 590 over just one 580 for sure. If you want to purchase 2 GPUs right away i would look at 570 SLI (for only $600-700 for both), which will give you excellent results at almost 580sli level. Also if 3 monitors is a sure thing then the more memory per card the better.
Yes the 2 570's will be somewhat close to the 580's but I don't think the 570's come in 3 gb version. I guess it comes down to what you want and what your willing to compromise with. Sometimes it just comes down to bragging rights to have the top model when the number two will suffice. You can get an Evga GTX 570 for around 319.99 on Newegg.