Generally, the quality of the RAM is the limiting factor, not the heat. Decent DDR RAM on GTS boards should do about 400-410. I run my Prophet II 64 at 400 and the sinks never even get warm. If I crank it much higher, say 410-420, I start to get artifacts, but the chips still aren't hot. Most GTS boards don't have RAM heatsinks at all and many can OC as high as the Prophets, which do. Take the Annihaltor 2 that Anand OCed for example. If you're really concerned about heat, put a slot cooler next to the card or leave the adjacent PCI slot open and blow a fan across the card.
You're simply not going to get much past 400 with 6ns DDR RAM since that already is a 67MHz OC. You have to get a board like a GTS Pro with 5.5ns or better RAM to get faster.