Just save for a mobo and AGP card. Trust me, it will be worth the wait. Same decision happened to me. I have the ATI Radeon 9250 256mb PCI and it plays games okay enough. On the graphic intensive games, I usually set it to 800x600 resolution and medium settings. (Half-life 2, CS:source, Halo).
If you want a PCI card that'll serve as a temporary solution, get the ATI Radeon 9250. Don't get a PCI under 256mb, otherwise, its not good. I wouldn't recommend a PCI anyways because of the choppy performance fpr the newest games.
An AGP card is like 5 times better than a PCI. PCI has little bandwidth for extreme graphic video scenes, which you are obviously going to face in some parts of a game. A drawback in getting PCI is that nobody has made it's graphics higher than DirectX 8.1. So that means that some things like invisible characters that look like "invisible but shiny goo" will not look right.
So, just SAVE money for the mobo, an AGP card, and possibly a NEW PSU!!!!!!! Don't forget, since your dell diemension has no AGP when you bought it, they probably made the PSU with little wattage. A decent PSU to run a good AGP IMO is 350W minimum. Your PSU is probably 300W or less. Look off newegg for any ATX PSU 350W or better. Using an AGP card on a PSU smaller than 300W will probably fry all the components inside your computer and that would suck badly.