You can to some extent. The 7800's (or atleast my GTX) came with a pretty pissed off cooler. It's always risky to OC with stock cooling. I put on the NV Silencer by Arctic Cooling, mainly for noise reduction, but also for better cooling. I actually bought it before I got the card and when I opened the box the first thing I thought was "sh!t that is one hell of a stock cooler! I could actually leave this thing on if I had to." Like I said, I don't know what the GT comes with, but the stock cooler on the GTX probably would have been fine, especially since the 7800 GPU runs much cooler than the 6800 anyways.
Google this and find somewhere that has a table of temperatures with stock coolers vs. aftermarket coolers, those are always helpful.