I switched over to using HDTV's a few years back, and am VERY happy I made the switch.
Although a purpose-built computer monitor will have a better picture, the image quality of an HDTV is fine for Sonar, AND you can get a LOT bigger display for the money by getting an HDTV.
My current setup uses 2 displays - my primary display is a 46" HDTV, and I keep the Track View open on that one, and my secondary display is a 32" HDTV. I spent $360 for the 46" HDTV, and $199 for the 32" HDTV, which are pretty normal prices these days for TV's of those sizes - with a little looking around. The 32" HDTV has the Console displayed on it, for easy access and viewing of the channel strips.
I USED to use 2 32" HDTV's. When I picked up the 46", I then had 3 displays, but eventually gave one of the 32" displays away, as I just didn't need that third display.
I'm LOVING it.
Bob Bone