You cannot overclock the 7900gs in the inspiron by normal (software) means. You'll need to flash the bios of the gpu itself to run it at higher clock speeds than normal.
You'll need to download an iso which has the files necessary for burning a boot disk. There is a user from NBF named Joker 5150 who has created an awesome bootdisk for flashing the gpu bios. You should read through some of the thread here:
http://www.notebookforums.com/thread157393.html
The download link for the boot iso is there as well in the first post. After you get the iso downloaded, extract it and burn it to a cd. You should have a ton of rom files with numbers in front of them, ie: 4801100.rom, which is what i run my gpu at. The numbers correspond to the clockspeeds you'll be running.
To flash the bios itself, reboot your computer, and on the bootup screen, hit f12 to start the one-time boot sequence. When prompted, select Disc Drive, then select the Max Memory option. Another prompt will appear, just hit enter.
Now you should eventually see R:/ appear. You can now type in the commands you need to perform the overclock. The format will look like:
R:/nvflash 4801100.rom
typing that in will begin the process of flashing the bios with the 4801100 rom file, you can type in whatever rom you want to use. There is also a file named stock.rom which you can use to return to factory defaults. For reference, the stock clock speeds are 375/500 (1000).
Hope this helps. As a disclaimer, you can ruin your gpu through overclocking if done incorrectly.
Problems? See this thread at archive.org.
