You have a pretty good video card as it is, first off I would get the A64 FX-53, if I were you.
Intel P4 2.4B, MSI 645E Max-U Mobo, 1GB DDR333, ATI R9800PRO 128MB, TT PurePower 420W, SB Live 5.1, Samsung 120GB, Maxtor 120GB, LG DVD+-R/RW, WIN2K PRO SP4