first step would be to find out what types of hdd your mother board supports and what hard disk is installed already. i doubt you could go wrong with a maxtor or western digital one though. get an ATA/133 and 8mb cache and 7200rpm and it *should* work in your pc. i have an ata133 hdd in my machine that only supports ata100 and its perfectly fine. they seem to be backwards compatible but im not up to spec on hard disks.
sisoft sandra should help you... has a welath of diagnostics in it