This processor replaced my old Pentium 4 3.0 Ghz processor. I didn't really need the extra processing power but my P4 would get incredibly hot when CPU speeds were idle. I can happily report that the E6600 runs really cool. It usually stays at 40-45 C but even running it hard with 3d games it doesn't ever get above 50.
I use this processor on a system that dual boots both Vista and XP and it runs both equally well. I am using 2 GB of ram. The biggest difference that I can notice between this and my P4 is the dual cores. I play a lot of games and I can run two graphics intensive games simultaneously without any loss in framerate. I can encode video or burn a dvd while doing other things and it doesn't even seem to notice at all. So far this processor has taken everything I've been able to throw at it.
As time goes on and the operating systems are optimized for multiple cores there may be a bigger difference between the quad core and the dual core but for now this baby seems to work great for everything. I highly recommend it as an upgrade along with a nice motherboard, some good ram, and a killer graphics card.