I have not experienced any problems with vista 64, but I only use software that supports it.
I'm using a dell 3008 WFP (30") and again, no problems so far.
Did you check out the nVidia link that I provided? That's a problem with a Viewsonic, but your Dell is the same general type of monitor. HDMI interface, etc. If you haven't already done it, a call to nVidia support might be informative.