I have a new PC with 2 x 235 GB discs configured as RAID 0 (stripe)
All works Ok.
My issue is that Adobe Photoshop for optimum performance requires a scratch disk that should be on a separate logical drive to OS.
Ideally a different physical drive.

Can I partition a slice on the RAID array ? ... if so can anybody tell me how to do this ?

Was your computer configured as such when you bought it or did you create the array?

Is this XP?

Windows only "sees" one drive: correct?

If so you can do it but will need software to do so.

I suggest: GParted

This is VISTA Ultimate.
Came as RAID 0, and you are correct PC sees one single disk.

Then you should have no problem in doing this.

There are GParted guides here:

http://gparted.sourceforge.net/documentation.php

See if they answer your "how to" questions.