First of all there is a 32bit Athlon XP and a 64bit Athlon 64 but no such thing as a XP-64.
Athlon XP's are some of the best when working with office apps, even better than P4 and 64's and cost about 30% less!
Find an ASUS or ABIT mobo, and dont get less than 512mb of memory. He'll see a huge boost in performance when going from 256mb to 512mb.
Athlon 64 3000+ Venice @ 2.25GHz
EpoX 9NPA+ Ultra
2x512mb Patriot XBLK
256mb BFG 7600GT
Samsung 200GB 8mb
500W Apevia (Aspire) PSU
Logitech Z-3 2.1
Windows 2000 PRO
3DMARK05 - 6***