What motherboard do you have?
Honestly, you could probably get a Get a GTX 280 or GTX 285 and an i7 920 (if they weren't discontinued) with that,
maybe even a motherboard (if yours isn't the right socket, but if it's not then you might need different RAM too and that'd be getting too expensive). The most you need is a GTX 260 (which is about like a 8800 GT but with newer technology).
But, now days you really want the newer tech with DX11. You can hardly even get those now days though because they're outdated tech with DX10. So, that means you want the GTX 4xx series.
What you want to look for in a graphics card is basically the technology on it, so DX9, 10 or 11. Higher is newer and is the better choice, as more games will start using it in the future and you'll get the best out of your games. It's best to future proof your purchases, so DX11 is the best choice right now. After that, you want the highest vRAM (video RAM) on the cards as you can afford (768MB, 1280MB etc. is what that generally looks like). I wouldn't get less than 1GB of vRAM now days, so...
If I were you I'd get a
GTX 470 (for $270 it's well within your budget) and you want to get it from a reputable company like eVGA, they have
very good warranties in case anything goes wrong and generally don't break. With NVidia's GeForce series the #xx doesn't matter, the first # is the series, the higher one's lately are newer. The second x#x is the quality of the card, higher the better. So, GTX
480 is a higher end card than a GTX
460. Generally anything 7 or above are always good. 6 and below are the lower spectrum and probably cheaper but not as great.
Each series generally gets better, and they have newer tech, so you'll always want the newer series. Just whatever you can afford in that series. The cards are so far ahead of games already though that no matter what you get, it's probably going to be enough as long as it's one of the newer series GTX 2xx/4xx.