Overall I'd say the P4 is a slightly faster CPU, but the XP 3000+ is quite a bit cheaper. For gaming though, they are very close and by far your main bottleneck with either of these CPUs is going to be the video card. They both have the necessary power to push a Radeon 9800 Pro or GeForce FX 5900 card near its limits. There are certain games that are faster on P4s (Quake 3, Ghost Recon, Jedi Knight 2) and others that are faster on Athlons (Serious Sam 1 & 2, Unreal Tournament 2003), but overall those two CPUs are close for gaming.
You are going to need DDR400 (PC3200) memory for both for best performance. AMD recently adjusted the speed of the 3000+ model to better compete with the 'C' (800 MHz FSB) P4s. The original 3000+ was a 2167 MHz 333 MHz FSB part, the new 3000+ is a 2100 MHz 400 MHz FSB part and is significantly faster than the old 3000+ overall.