I'd agree with conceptumator, you'd be able to upgrade RAM, hard drive and maybe a new video card. That would be about it. You can check crucial.com and it will tell you what RAM you can use and the max the PC can take. You can also do something similar at support.dell.com using the Dell Service Tag on the back of the PC, but Crucial will give you WAY better prices. Usually.
Cheers
Rob
The answer is always "PEBKAC!