Hmmm, I've had a few HW disappointments in the past. But, here's by far my biggest disappointment.
I got a 386SX 16MHz back in 1990. It came with 2MB of RAM. About a year later, I boosted it up to 8MB (which was like a $300 upgrade back then!). With all of that software typically requiring less than 2MB to run back then (and the fact that Windows 3.1's memory management SUCKED), it made very little noticable difference. I really never saw dramatically faster performance until I bought a new 486 machine.
The S3 ViRGE (Diamond Stealth 2000, if I recall correctly) I had with my P200MMX comes to a close second (before I added a 4MB Voodoo accelerator to it).