It's amazing how fast and responsive a clean XP install feels, even on dated hardware. I've had great results on P4 class machines at 1.6-2.8GHz. I even ran for a very long time on a P3 equivalent AMD at 800Mhz. Clock speed was limited, but the machine was rock solid. You don't need a lot of memory either. 256-512M seems adequate.

Just avoid the temptation to install apps, browsers, etc.

Steve