Click the Video tab at the top of the page, and look at benchmarks of Doom3, Half-life 2, and Far Cry since those use "modern" 3D rendering more than most others.
Something to consider is what screen resolution you want to game at. The $150-200 cards do great at 1024x768 and in some cases at 1280x1024. The $300 cards are solid at 1280x1024 and sometimes 1600x1200, but for consistent 1600x1200 you need the top of the line cards or to wait for the next generation of midrange cards.