Vista will play games designed for DX9 and XP just fine. You won’t have problems in that regard.
And the drivers for Visa have improved to the point that gaming is virtually identical to the experience in XP. It was slower in the beginning.
If you already have a copy of XP that you can use in your new computer, I’d say go ahead and use it, just install SP3 and newer updates and you’ll be set as far as the operating system.
If you are buying a new operating system for a new computer, then I would advise getting Vista 64 bit. I disagree about driver availability on new equipment…it has improved immeasurably since the introduction of the operating system. If you have older equipment, especially some peripherals like printers, that is where you will run into some issues with Vista 64. On a new build, I don’t think you will have problems with drivers for the main computer.
The main advantage to a 64 bit operating system is the amount of ram that you can access. A 32bit OS will utilize less than 4gb, due to overhead. If you don’t foresee putting that much ram in your system, ever, then stick with the 32bit systems.