Quando,
The only thing that you possibly need to upgrade would be the Graphics card. Check your manual or call Dell and see what the upgrade path would be for the graphics card (PCI-E or AGP).
Then go to OMP(Ohio Model Products) or AFPD's website and look at the specifications for the graphics cards that will work with AFPD. They have a good list of recommended grapic cards.
The video card is the weakest link in most systems that you buy from the big name computer manufactures and are the first thing that I recommend for people to upgrade when
they purchase them. I would go with at least a Geforce 6600 with 256 meg of ram. They are pretty resonable priced but also look at the higher end models too.
Just my two cents worth.
John