I run vista 64 bit as well and it works just fine...loads under the Program Files (x86) folder and loads right up. i have an 8800GTS and it does lag a bit at times when your pushing games to the max. Crysis looks amazing @ 3840x1024 but I have to run it on low settings since so much is rendering...
on the monitor part, my suggestion is keep your 24" and pick up some of the Dell 22" (or the gateway 22"...just as nice and look neater too)
then run the TH2Go off of one DVI @ 5040x1050 and the other monitor @ 1920x1200. then you can get you one of those LCD mounts and mount it on an arm so you can swivel and turn it portrait or lanscape...did that with my extra 22" and it works wonders...
Are you using a switch to go between TH2G and the standalone??
if so, which switch?