This has been explained a thousand times, but here goes 1001
64bit processors allow for more bandwidth making everything faster. Until the AMD64, 64 bit was exclusive to server chips like the Itanuim and Opteron.
64 bit the future, but nothing that runs on a desktop currently uses 64 bit, everything is 32 bit. Windows is currently developing a 64 bit Windows, but I don't think thats do out till 2006
The AMD 64 FX-51 is AMD's flagship CPU and blows away standard P4's due to its on-die memory controller, large L2 cache and such. HOWEVER, Intel trumped the release of the FX-51 with its P4EE, which has a MASSIVE 2mb L3 cache, so in total 2.5mb of cache
The FX-51 and P4EE are pretty much neck and neck, but most benchmarks favor the P4EE as the winner, it all depends on who you speak to
Not that many of us can afford either as the FX-51 is $700 and the P4EE is $1000
So for us poor folk, its between the 3.2 P4 and the AMD 64 3200+(though they just released the 3400+, which is marginally faster than the 3200+), to which there also pretty neck and neck, and the winner depends on who you talk to
You really need to look at the benchmarks for both, check your budget and decide for yourself on which is better