Going from no sound on my 486 SX25 and adding a Sound Blaster 2.My first decent soundcard with ASIO drivers.
Using audio programs such as Rebirth gave me 7ms latency instead of 70+ms!
The least impressive was going from a P4 3.6 to my E6600 Quad.
I honestly can't see any difference after nearly a year.
In fact my P4 was faster at a lot of things.
Beige case to Coolermaster Stacker case.
Upgrading from my Dell 22" to a Hyundai 24". Sounds weird, I know.