My advice is get a SINGLE GTX680.
2xGTX680 is pretty much overkill unless you have a triple monitor setup.
If you are considering GTX680 SLI, wait and get a single GTX680 4GB card. If it's adequate then it wasn't much more expensive (the extra VRAM likely won't see an advantage in any games), but if you go SLI you need the extra VRAM.
2xGTX680:
If you are certain you'll go with this way (not recommended) then wait and get a GTX680 4GB version. SLI and Crossfire setups do not add Video RAM, it's cloned. That's the reason the HD7970 has 3GB, not for single card but for Crossfire and you can see the improvements in several games if you compare a 2xGTX680 2GB vs 2xGTX7970 3GB. You can observe a VRAM bottleneck for the SLI configuration.
RESOLUTION and game setup:
I have a 2560x1440 monitor. I've discovered that for most games it's actually better to use 1920x1080. Basically I get an IDENTICAL image but slightly better anti-aliasing and a lower frame rate.
You can experiment, but here's my rule of thumb for the GTX680:
1) run FRAPS and then the game
2) aim for an average of 70 to 80FPS to minimize sub 60FPS dips
3) if FPS drops below 60 and you have 2560x1600, lower to 1920x1080
4) compare 1920x1080 vs 2560x1600 for quality (Max settings)
5) enable adaptive VSYNC when done testing
*Basically a single GTX680 can max out pretty much every game on a single monitor, and there are a LOT of advantages over previous gen cards so definitely get the GTX680.
Read a few reviews and look at the benchmarks.