The one I've got (not physically, it's for a customer) is running XPSP2, a P4 3.2GHz and (luckily) has 3GB RAM fitted. It's using the integrated Intel 945 chipset (ugh) and the customer wants to install our 3D simulation application on it.
The machine seems to support PCI-Express x16 so I think it's more a problem of space & power in this mini-tower. By modern standards we don't need a top gaming card so I am imagining $100-150 should get something quite good enough... I have a preference for nVidia but really something that doesn't come plastered in fans and is small is a priority, I know they exist.