You'll do better to build (my preferred route), or buy new. Your upgrade options are either limited - slightly more processor if you get a used P233mmx for about $30, or more RAM (not much point in going above 64mb in a mobo with an FX or TX chipset), faster larger hard drive, and a clean install of you OS & software; or ridiculously expensive with overdrive chips or computer-on-a-PCI card upgrades.
Neither of these is going to get you a cost-effective major boost in performance, if that's what you're looking for. The set of small upgrades can get you a computer that feels a whole lot faster, provided you are only using it for browsing/email/basic office apps, and you don't want to install WinMe or W2000.
If you want real power you'll have to build, or buy new, and your choices are myriad. Someone else is bound to jump in here with specifics, and there are lots of guides on budget vs. mid-range vs. high end on AnandTech and many other sites.
My $0.02