A GTX 460 will be a roughly 20% upgrade from two 8800GT 512MB's in SLI. It won't be a huge difference and it might be more worth putting that money towards a completely new system (you can reuse case, DVD, HDD, PSU, for now).
If you have a Microcenter, you can pick up an i5-2500K, P67 motherboard, and 4GB DDR3 for $350. It won't do much more than a 5% bump for graphics, but you'll have a system worthy of all the best new cards.
Also, if you just want a graphics card, consider the $180 (After coupon & rebate) Radeon 5870 on Newegg right now. It blows all other graphics prices right now out of the water.