I've got my 128 meg Gainward Ti4200 running happily at 310/560
From 275/520 I increased the memory at 10mhz at a time, ran 3dmark benchmark, looped the demo for a while, played some games, and surfed the net -- always keeping a close eye on the screen for graphical anomolies. The following day I would increase it by another 10mhz. At 570mhz I got some tearing in Dragothic so I dialed it back to 560. I got the same tearing at 565 so it looks like 560 is my max. After determining the max on the memory, I did the same thing with the GPU. However, I have yet to determine the max on the GPU. I'm currently at 310 but I haven't tested any higher yet. I wouldn't be suprised if I hit 320-330 but at the very least I know I'm rock stable at 310.
I gotta say I'm pretty happy with an almost 30% overclock.