blackductape, 12 audio tracks is amazing. What kind of audio tracks are you talking about? It doesn't seem possible with your specs. Also, the number of VST depends on the RAM and the type of VST. Some VST are very CPU intensive - loading one VST is sufficient to max out your resources.
As for dual booting, it should the standard method (althogh I've not used Vista myself). First, you need to partition your HD using the Windows installation disk. You can have as many partition as you like, but each version of Windows is to sit on a different partition. Then install XP first - on one partition. After you've done that, inserting the Vista installation CD should prompt you to whether you want to dual boot it in a different partition. Just follow the instructions. After all is done, on boot up, you get a screen giving you a choice as to which version of Windows you want to boot.
Other way to do it is to get a program like Partition Magic. I don't advise it as it takes up resources and runs in the background. This affects the DAW.