i agree on the 4870, its probably one of the best bangs for your buck right now...its performance is right up there with the 280 to be honest, and a whole hell of a lot cheaper.
The 4870 IS the best bang for buck card currently. There are more powerful cards however and some are also great deals. Can't go wrong with a 4870 tho, even 1 will destroy your duel 8800's so you won't be disappointed
If you're kinda tight don't even think about spending that money. It's not worth it. With 2 8800's you're fine. I have a 9800gt and it destroys source engine games. 1 is plenty enough, now 2?! For sure you're set.