hmm tough one..
probably my K6-2 400 CPU.. went from a P133 to that without the change in mobo!
after that, I'd have to say my Voodoo Banshee (got it when I still had my P133, it let me play most games very well), Voodoo 3 (for games now adays), and my extra 64megs SDRAM.. having 96 megs is a good thing, and I only wish I could up that to 256...
ooh.. another good one.. Upgrading my hard drive from an old 200meg HD (I think it was from Quantum), to a 4.3 gig FUJITSU hd.. that was an amazing performance increase (even though it has NO cache), and I hope to get a jump like that again soon..
ahh then comes the money problem..
EDIT: error in my RAM number!