You can get a nice quad for $700-1,000. By toggle with one monitor do you mean keep one monitor connected to 2 computers? If so, this is possible. With the right mobo, upgrades won't be an issue .
The best thing to do would be to build it as if it were going to handle video or would be used for gaming, but still supported by digidesign as Pro Tools will most likely be your daw of choice and most picky. Also, no vista of any kind. Stick with xp pro thats optimized for performance and not for visuals (shading and all that other fun colorful shit) and you'll be set.
Let me know when you want me to start looking for parts and I'm on it.