I wouldn't bother buying Windows Vista for your current PC, when Vista comes out it will be cheaper to buy a new computer that comes with Vista (OEM) than it will be to purchase the OS in its retail form.
Besides, I don't think they're bringing out any Vista-ready AGP cards anymore, like I said, the technology is too outdated. It is now PCI-Express that is taking the lead. This doesn't mean that you can't install Vista on your current machine, though.
What kind of videocard is currently installed?