DO not touch the X1600. Its a poor midranged solution compared to that X800 or a 6800GS for that matter.
As for the OC'ing. The IceQ design is quite similar to the Silencer design.
My X800pro went from stock (475/900) to 535/1125 ---or something to that effect. I cant remember the exact frequencies, but it was very close to that.
So I wouldnt hesitate personally.
Nice setup overall....but like I said, try to stay away from midranged ATI cards that are x6 or 700 and the x13, 1600.