Needing to know a decent GPU for my current setup...

OK, guys, I built myself a machine for my birthday earlier this year. Specs are as follows:

Motherboard: ASUS Rampage II Extreme
RAM: 6GB - Corsair Dominator 1600
CPU: i7 920
Case: HAF932
PSU: 750w Corsair

...and everything else that isn't really necessary to list for the topic of this thread...

Anyway... I bought the cheapest video card I could find, which happened to be the 3870 by Sapphire. It is alright, of course when I bought this the 4870 was the top of the line. I only spent $60 on this card. Like I said, it's a good little card, but I want to do more gaming. For my kind of setup, on a 40" display running at 1920x1080, what would have a decent framerate at that resolution with pretty good quality? Cooling isn't too much of an issue. I kind of want to keep the price below $400 and as for SLI/Crossfire, I would like to stick with one card if possible, but if there are any really good price-to-performance sets, I'd like to know.

Furthermore, I was at first thinking of the GTX 295, then I was thinking of spending less and going with GTX275 in SLI, but I don't know. I was looking at performance comparisons between one 295 and multiple 275s and 285s and I have found that one GTX295 would be better for the wallet and better for performance because to get that performance with other said cards, it'd cost a lot more. Maybe it's just me...