You have a Dell business machine, but with your model, there is no reason for not having some good gaming. Your Chipset, from what you have furnished, should be the 915G which your particular one came with the new Intel Integrated GMA 900 graphics that is very good, much improved over the Intel Integrated Extreme Graphics. Your 915G chipset also allows a PCI-Express card which replaces the AGP card. Chipsets with PCI-Express slots do not have an AGP slot.
Newegg.com has a very large selections of graphic cards and is a reliable company. At http://www.newegg.com/app/manufact.asp?catalog=48&DEPA=1on the left "Advance Search" under "Interface" select PIC-Express. Scroll down to price and in MIN type $90, MAX $125.
My suggestion is the Nvidia 6600 PCI-Express at $119 which performs better than the ATI Radeon X600 Pro PCI-Express.
Got the above with the 2.53 ghz. Celeron, 512 MB DDR memory, open AGP slot. Need your thoughts on a $100 or less(don't do serious gaming) video card. Would you reccomend a AGP or PCI type ? What do I have to do to disable the video controller after installing the new card ? Any other advice you can offer is appreicaited as I know little about video cards(other than the fact that some of the on-line games I do play are freezing up and aggravating the heck out me).