Reviving this old thread on the offhand chance anyone else is in the same boat:
I've had the original Inspiron XPS (3.4 Ghz P4, 2GB RAM, 256 MB Mobility Radeon 9800) for almost two years now, and was almost at the end of my rope as I saw system performance drop off drastically, with heat and noise increasing just as much, to the point where my colleagues thought I was plugging in a hair dryer

I tried all the usual suggested steps to update performance, and was still only getting around 2 FPS in SL. I eventually had to uninstall the client as it was completely unusable.
Then the latest release of the client comes out, claiming AGP support for additional ATI cards. Now while I don't exactly know what that entails, I thought I would install again to see if the little checkbox for AGP was still ghosted out. It wasn't. Woo hoo, I thought, but to no avail.
So that took me down the road of updating my drivers. After some research I read that ATI's Catalyst drivers for desktop machines can be installed with a 'Mobility Mod' (
http://www.driverheaven.net/patje), and that the end result was supposedly more stable for this system than using Omega drivers (if you're planning to go down this road, you might also want to read up on using 'Driver Cleaner' - its been mentioned in other technical threads
here on the SL forums).
However, while the driver seemed stable in the short time since installation, still no joy with the SL framerate.
After reading some threads on other boards from people equally upset with their expensive paperweights there were several suggestions that the hot-running XPS is set up to throttle its CPU speed back when the fans cannot cool it down properly, and to ensure that the fans and more importantly, exhaust grilles at the back of the unit are free of dust. Dell has service manuals online with instructions on how to remove the fans but its pretty easy to figure out nonetheless.
I'm embarrassed to say how much crud was back there. But the difference is UNBELIEVABLE! After restarting the first thing I noticed was how quiet it was now. The second thing I noticed was that I was now getting 35-40 fps! Between this and the new drivers (and perhaps the AGP support enabled in the client) it is like I have a new machine
