I think Hydravision will let you do that in XP, but not in 2K. nView should let you do so in both. I think it's established that nView is a bit better than Hydravision, but I don't really follow that feature.
Visiontek was reputed to have good 2D on its GF4 line, unlike many other vendors. Why are you considering changing to ATi? The only advantage I'd see would be better DVD playback, but you'd be taking a step back in most other ways (if your Vtek has signal quality comparable to ATi).