I second Greg's suggestion.
I use a HP i5 Win7 machine and it handles everything due to it's Quad Core. $680
To give you an example of how much these machines can handle, when I use mine I have NightSkiesNetwork running for my AstroVideo broadcasts which is very damanding, I also have Google open so I can answer questions I am asked while broadcasting, I have Cartes du Ciel running as my sky guide, I have a Mallincan Xtreme AstroVideo camera running, I have Mallincam camera Control software running to operate the camera (seperate to 'Video In' from the camera), sometimes I have music playing, and I often have open 3 or 4 other websites as reference for my answers to viewers.
I also open links the viewers post up in the Broadcast chat window, and to top it all off, I also have PhotoShop CS5 open and I take screenshots and save them in PS.
All these at the same time and it has never stressed the machine.
I am also about to add Autoguiding.
Don't get anything lass than Quadcore or it will struggle to cope with all the demands of running AstroImaging programs and cameras at the same time.
I went through this same exercise as you are going thrugh now, about which machine to use.
My problem was solved by Bert (Avandonk) who is familiar with running multiple imaging programs and cameras, so I am just passing on the help I recieved, and a damn good suggestion for a machine.
http://www.pccasegear.com/index.php?...ducts_id=20033