1. Upgrading from my Matrox Mystique (without bilinear filtering capability) to a Maxi Gamer 3DFX 4MB, and running Motoracer, Tomb Raider 1 and GL-Quake with 30+ FPS max details!
2. Going from a Geforce 2 MX to a Geforce 3Ti 500. That card ran everything at max for two years in 32bit colour.
3. From my FX5600 Ultra, which I was sure would be a killer card, upgrading to a 6800XT AGP and using Rivatuner to unlock the extra Pixel Pipes / Vertex Shaders (in 3DMark 2003, I went from 2400 ish to 11000). I replayed Doom 3 and went from 640x480 and low details to 1024x768 and high details still with double the FPS ^^