Voodoo 1 - The best upgrade of all time. Before 3Dfx came along upgrades were normally limited to a faster CPU (if you were lucky) or more ram neither of which made much difference to how games actually looked.
When I first plugged in my Voodoo and got it to work it was like all my games had gone under a radical form of plastic surgery they simple looked awesome.
I remember being a 16 year old with no idea whatsoever... i remember plugging in my Voodoo2 and putting the monitor into the wrong port, for months I spent time using onboard graphics without even realising... then the original Alien Vs Predator was released and I realised my PCs setup mistake... and did that game look great in its day