Why not go with an Athlon64 XP chip? The older 32bit Athlon XP's were typically behind the last generation of Northwood P4's in performance. The 64 bit chip isn't really that much more expensive and has the onboard memory controller. It runs 32 bit code faster than the older Athlon XP and will support 64bit programs as they come out.

Edit: Oh yeah, no graphics card?