If waiting is an option I'd wait. We're between generations right now.
AMD 64 MB selection is unimpressive. We're still on the first chipset silicon. The AMD 64 - 3000 is down to $215 USD so it and a MB would fit your budget. But, the straight 64 I think is a different socket size from the FX so upgrade path is iff'y.
The 800 MHZ FSB P4 (2.8-3.2 gig) w/875 chipset is a good choice and would fit your budget and outperform the AMD 64 unless you get the AMD FX chip @ $700 USD.
Also, the MB manufacturers are holding back waiting on the PCI-X generation of MB this spring.
The risk is always there that when MS comes out with the 64 BIT Windows it will have issues with the early hardware (MB or CPU). (It's happened before with the AMD 333 MHZ K5).
Bottom line is I'm waiting until late next spring at least. With my XP2000 I'm in no need of more speed and prices will go nowhere but down on the hot new stuff.