Using ATiTool, I got up to 450/325; however, I was using a Vantec Iceberq-esque HSF (it's a knock-off of the Vantec). Also, it didn't make any noticeable difference in games. I play Doom 3 and Morrowind. I got a higher 3DMark03 score, but that was all. Also, Doom 3 ended up crashing majorly. The screen turned black and then blue with a loud, low hum coming from my speakers, forcing me to reboot. So I turned it back to default and intend to keep it there, unless I decide to get a new card.
Note that your results may be better than mine, or worse.