I guess I'll add to this one, 512 in my laptop, 1GB DDR (PC2700) in my main pc, upgraded from 768MB PC133.
Actually, first I went down to 512MB but then scored a deal on another 512 and am running in Dual DDR mode on my Nforce2 board.
To be honest, didn't noticed any difference between 512 / 768 / 1024 in swap usage, as most of my apps run in the 256-384MB area. I guess some games might use more but I'm not big on games lately. I'll probably split the ram and build a second upgraded box w/ the other 512DDR. But for now I can enjoy the bliss of 1GB.