For good performance but not the ultimate for game playing I like the GForce 2 MX 400 64MB range such as this one from Dabs at £25 click here. If you don't have an AGP slot, then there is a PCI version but it is quite a lot more expensive at £40 click here both have 64M ram which will lighten the load on your processor ram.
will be whether or not you can switch off the onboard graphics. If you have the mobo manual you can look it up if not go to the mobo manufacturers site and download the manual. If the mobo does not support switching it off forget it and wait until you change your pc.