I have a P4 rig still on AGP. Just recently purchased the above mentioned card from OcUk. Its a massive performance increase over my 6800GS. Just make sure you have the juice to run it. If your brother has an old P4 rig he could have an equally old PSU to go with it. 30A on the 12v rail should do it. Or 16A on 2x rails. My old psu was an Enermax 485w with 2 rails, 16A on each. I've heard people running these cards with a lot less though...All depends on the system.
After you get the card run 3dMark05 on default settings and you should get about 7000-8000 marks. Or 3dMark06 around the 4000-5000 marks area. If you get a low score around the 2000 mark it could be because of the PSU. NB...These are just average figures, please dont post that you have more...its probably due to your superior system and overclocking skills, along with the latest drivers etc etc etc....