nice doesn't really do anything with respect to I/O.
Yes I tried it and it never made a diff from one end of the spectrum to the other:)
The best way to control I/O in this manor is to physically isolate it from the rest of the system(be it on a different controller connected to different disks etc).
Well, not always possible!
I am going to try Peter's suggestion of ionice tonight.
Thanks everyone! jlc