Don't think that the framerates you see on the benchmarks are the same ones you're going to see in-game; The benchmarks are much much more intensive than the actual game is, so the framerates are a lot lower.
For videocards, you buy the best you can afford. If it doesn't run like you want, you have to tone down the settings, nothing else to be done about it. Luckily even the 7900GS is a powerhouse of a card that'll play most games out there at max. Oblivion is obviously an exception, but the second most-intensive, FEAR, will easily be maxed out.