I'm noticing a problem with the way you're shopping. You're basically saying "I want a great card but I don't want to have what it takes to run that card." It's somewhat like someone coming up to me and saying "I want a ferarri, but spending more than $50 is not an option, so tell me how to do it without telling me it can't be done."
Your best option is to find a relatively inexpensive motherboard that will run a PIII and that also has an AGP slot. You can get one dirt cheap. Assuming the Dell case is a crappy one that isn't ATX compliant and therefore the new motherboard won't fit in it, another $30 would buy you a case.
My point is, if you want to have good graphics, you will require a good video card. That's just hard to come by in PCI because no one that wants good graphics is interested in buying a PCI card. You'd be much better off getting AGP capability, and THEN getting a card.