First off, you have to determine what you have now. From the documentation I have seen:
http://support.dell.com/support/edoc....htm#wp1076752
you have a Pentium 4 Socket 775 motherboard that uses DDR1 ram. I don't know the speed of the cpu, or the speed or amount of ram you have.
There is a replacement motherboard at Newegg for you:
http://www.newegg.com/Product/Produc...82E16813157115
It can use either DDR1 or DDR2 ram, but not both together. It does not have onboard video, so you would have to buy an AGP video card to go with it. This is not the best solution, since video has moved on to the PCI-E slot on the newest cards. As a budget/intermediate build it might be acceptable to you.
The best available AGP card that I know of is the ATI 3850, but it is an expensive investment in an obsolete format..
http://www.newegg.com/Product/Produc...82E16814131090
A reasonable compromise might be this 7600GS:
http://www.newegg.com/Product/Produc...82E16814130076
It would probably run with your current power supply.
But there are usually problems getting a regular atx board running in an oem case. The connectors for the on button, restart button, etc will be a pain in the butt. And it might not fit perfectly. Hard to predict that. If you can find a cheap case locally, that might be a better plan.
I still advise waiting till you can afford a better, modern system, but what I've listed here is possible, if not the best solution.