Guillemot International 3Dfx Voodoo 1 (4MB). Got it for Xmas from Electronics Boutique for £99.99, I think I was 12 at the time. We couldn't get it to work on Xmas day, and I was absolutely gutted. I managed to find a fix described in the technical section of a back issue of PC Format or something and was able to get it working by adding something to POST with a floppy disk. Me and my dad couldn't believe it worked. I didn't talk to anyone all Xmas after that.
It was amazing going from plain old VGA to 3Dfx. I remember trying it out on the first Tomb Raider and being amazed, then installing every demo I could get my hands on, like Turok, Wipeout, POD etc. Quake 2 was just unbelievable. Coloured lighting!
Sadly it was let down by the terrible 686 cyrix processor we had though which just couldn't handle floating point calculations, so frame rates were terrible on a lot of games, but I didn't seem to care back then.
Then I had a crap Emachines 900Mhz Celeron at uni with onboard Intel graphics, Played MaAx Payne well enough though.
I "upgraded" it to a Radeon 7000 64MB for about £40 so I could play Ghost Recon, which was probably the most useless graphics card I've ever had.
Then I got a 6800 vanilla around the time of HL2/Doom3/BF3 and that was AMAZING. Probably the biggest jump in graphics fidelity of the last 10 years.
Radeon x1950 Pro - OK, not great, but let me play Oblivion
8800GT - pretty good, still couldnt run Crysis properly though. BF2 was great though
Radeon 5850 - Awesome, still couldnt run Crysis properly though...
HIS IceQ x2 Radeon 7970 - Unbelievable - destroys all games, but there just isnt anything that interesting out at the moment that will stretch it.