32bit XP is more than capable of running the latest DirectX9 games. As long as XP isn't crashing due to errors caused by the unused RAM, then you'll be ok sticking with 32bit.
Good news about getting the game reinstalled. Were there hidden registry entries?
If the calculator is saying your system needs 660W, and that's including the extra 30%, then you'd better go for a 750W to allow for future upgrades. A 650W would be too close to the limit.
Thermaltake Toughpower 750W £67. I have the 650W version and it runs an 8800GT with stable voltages and no overheating, even when overclocked. Highly recommended.
OCZ GameXStream 700W £73
OCZ ModXStream 780W £91
Coolermaster 700W £89
Seasonic 700W £108