I'd just go with building another complete system. You could probably save your video card, maybe sound too, but I'm betting you'd have to replace everything else. If you're going for a decent system don't settle for SDRAM, skip on to DDR for an AMD system and RDRAM for an Intel.
Prester, do you remember the specs on the P4 533fsb system? If it includes OS, 1066 RDRAM, 7200 HD, and a decent vid card I might give one a shot.