The 9000 series is not top of the line. It's a little behind the times, actually. It would be like a GeForce 6600 or so. Even going up to an x-series would be a big improvement, card-wise. Keep in mind the 9000 series cards have been around for years. "Years" and "performance" rarely correlate with each other when it comes to gaming
Here's what you do: Look for a motherboard that has BOTH PCIe and AGP. I found one for my LGA775 chip, but I know there are also AMD chipset mobos with the same options. They have both DDR1 and DDR2, AGP and PCIe, but you only use one or the other at a time. This way you can upgrade slowly without having to get a new CPU, a new stick of ram, and a new GPU all at the same time. You can just get the mobo, then when you have some extra $$$ get the vid card, then the CPU, then the RAM, then when you want to go to a proper mobo down the line you have all the parts up-to-date, and can just swap it out (or keep it, whatever).