I mainly got an alienware case so I could stick a couple aditional fans in it to keep my system cool.
My computer litterally runs about half-as-cool as my previous computer did. The old one typically ran at about 60 celsius, and when it got really worked up from me doing 3d work, or blowing s--- up in UT2004, it got to around 85 celsius. Now, with an alienware case and two extra 60 mm fans, it runs at about 27-35 celsius, and the worst I've seen it when it gets worked up was about 47 celsius...
Still, that's cooler than how cool my previous system ran, when it was just running; not doing anything (did that last sentence make sense?