My biggest jump...
486DX2 66 -> Celeron 300A oc'd to 450 on a DFI board, all it took is 1 jumper... no messing in bios... no voltage... nothing.. just 1 jumper..

And i loved watching that celeron 300A beat pentium 450's due to the much faster cache it had when overclocked...
NOW THAT WAS FAST!!!!
now the most memorable PC upgrade i had:
1. Sound Baster AWE32.... OMG!!! if u recall those days where onboard sound was not available... Sound Baster PC's were Ownage especially in wing commander.
2. 3dfx Voodoo ..... Yes i was an early adopter of SLI... this is why i cant get myself off SLI even tho it is plagued with problems like micro shutter, and drivers.
But my systems have always looked simular to this.. XD