best bet is going 2nd hand due to the above, also price up a 2nd hand hard drive in what ever size you need and a cheap enclosure.
you considered an upgrade to 7 rather than a down(upgrade!) to XP from vista? you would be surprized how well 7 runs on older hardware. i tried it on my old IBM T40 1.8ghz pentium M with 1gb of ram and it ran brilliant! granted not quite as good as XP though, but i recon with 2GB of ram it would have been sweet! i still use XP on it though! just cant justify a ram upgrade as i rarely ever use it these days!