I have a Dell Dimension 3000, which is pretty much the same system just a different version. I to am stuck with only PCI slots. It is not recommended to get PCI graphics, because of poor bandwidth and just the fact that PCIe x16 is better and even the older AGP is much better.
So I bought a ATI Radeon x1300 which is probably the best core for a PCI graphics card. Its worked great for being PCI. Can't play anything on high settings, but played games like Fable, Need For Speed Underground 2, World of Warcraft (runs excellent), and Warrock. the only complaint with it I have is it goes slow playing Guild Wars for some odd reason which no one can figure out. My other complaint about it which is also why PCi is bad is my friend jsut bought a PCIe x16 graphics card for only $20 more and its easily 10 times better. Here is the exact card I got:
ATI Radeon x1300 Hope this helps!
Edit: I should also add if your looking for something cheaper perhaps getting an ATI Radeon 9250.