Going from an old ass P2 windows 98 pc with 64mb of ram (Which for some reason had a 32mb video card too). To a P4 xp machine with 1gb of ram.
Having a pc where it didn't crash every time you tried to open up more than one thing in internet explorer was a godsend, or you could have just msn messenger or just internet explorer open, christ knows why it had a decent graphics card for the day, you couldn't open a game on it if you'd tried.
Dialup to broadband, that was even better, the amount of porn I downloaded that day is indescribable.
After that to be honest every upgrade after has just been, wow thats much better, but thats what I expected.