You can use any card that is physically compatible with your motherboard. The operating system you have installed will only limit what functions are available to be used. Since you're using Windows XP, you can use a 5xxx series card, but you will not see any of the Windows 7/Vista specific functions (DX 10/11, Eyefinity).
you can run any of those cards on XP, you just won't have the option of running the DX11 API as XP doesn't support it. The 4850 is probably the best price/performance you'll get though, unless you need more power than that and want to go up to the 5770 or better.