Being brutally honest here. I advise you sell off the machine as it is in its parts form and just buy a new one. The processor isnt that desirable these days nor the graphics card. Plus it is next to impossible to find a AGP graphics card these days. The card you have is about 4-5 years old and with PCI-e it is pointless to upgrade as the improvement would only be seen in spending a lot of money on second hand parts as new ones are hard to come by and not worth the money.
You can get a pretty decent dual core system these days for £500, the parts you have already are probably worth about £100 in total minus the hard drives. So thats £100 you could spend to getting a good graphics card on another system.