Go to the MOB/PC manufacturers website and download the latest BIOS if you don't already have it. Normally there is a description of all the BIOS updates there and you will probably be able to see what sort of HDD it will support. Or e-mail their TS and they will let you know.
if you want an ATA 66 HDD, you can get a pci card from
http://www.novatech.co.uk that will give you an ATA 66 bus controller and a couple of USB ports to boot.
the other option is a scsi drive, although faster they are more expensive and require a scsi card. I have read reviews that give the IBM HDD top marks, but they normally come as a bare drive and no disk management software, people like
http://www.smcdirect.co.uk when supplying a HDD include some disk management software. depending on what you want to do on the PC, having a super fast drive may not be necessary on your 200mmx as you wouldn't see any great advantage as the CPU probably wouldn't be quick enough to see any improvement. You would also have to look at other possible bottlenecks that you may have i.e. graphics card, before you would see any advantages of having the faster drives.