As a gift I'd like to help my 12-yr-old son upgrade the computer he built last year so that it can run Realflight G4.5 (a flight simulator) which requires a GPU with at least 512MB, Pixel Shader 3.0 and DirectX9.0 or better. The OS is WinXP with 3GB RAM and a 19" LCD monitor running at 1440x900. The motherboard is Gigabyte's G31M-ES2L and I'm not sure about the power supply, probably about 350W, so power may be an issue. What cards do folks suggest for under $100?