Resolution is important because there's no sense buying a card w/ more "juice" if your monitor can't support it. LCD monitors are fixed resolution so if you are playing at 1920 x 1200, you need more powerful card than if playing at 1280 x 1024. FPS is also game dependent, 50 fps in one game won't translate into 50fps in another.
You certainly don't need a high end card if less than 1920 x 1200. I'd suggest that you check out THG Graphics Rounduphttp://www.tomshardware.com/reviews/best-graphics-card,2404-3.html
The 58xx series has just hit the streets but the only one I have seen that you can actually order (and wait who knows how long for delivery) is the 5870 but at $379, it's almost double your budget.