Hi,
1) First of all it is SLI for NVidia and Crossfire for AMD cards when discussing multiple GPU's.
2) Secondly, Crossfire still has serious problems that haven't been solved including running at a PERCEIVED 30FPS in DX9 games (2nd GPU is basically barely used but registers as if it was in FRAPS).
3) From a VALUE point of view your best bet is just to get a SINGLE GRAPHICS CARD between $300 and $500.
4) You should also read about NVidia's G-Sync technology for monitors starting in 2014 (requires GTX600 or better graphics card).
5) If looking at newer AMD cards like the R9-290 get one with a good custom cooler (not sure if any are out yet).
6) The EVGA 780 3GB (967MHz base) is about $500 and would be at the top-end of my recommendation. The Asus GTX770 2GB is about $330 and at the low end (based on what you were willing to spend).
Summary:
My main point is just upgrade the graphics card only, but it really depends how much you want to spend. For about $330 for a GTX770 or a similarly priced AMD card you'd have a far better system.
If you were building today I'd tell you to get an i5-4670K, but just get the graphics card first and see where you stand in terms of performance. You can also look up benchmarks comparing CPU's for a particular game to see how much benefit you'd actually gain.
Also, if you get a constant above 60FPS then upgrading your CPU won't accomplish much if you run at 60FPS VSYNCH'd (recommended) as the bottleneck is now an artificial one in software to force to run at the refresh rate of the monitor. Again, it varies by the game.