I'm no software guru, but I believe that once software is multithreaded that it is up to the OS how many CPUs will be used. I think XP Pro and W2K Pro support up to 2 CPUs. For quad support you're probably looking at 2K Server or Linux.
Yes, Zemlin is right. You would have to use Windows 2000 Server --- Not Workstation or XP. Workstation and XP only support 2 CPU's. Windows 2000 Server supports up to 4 CPU's (maybe up to 8 CPU's, but im not definite on that)
Now, the real question is whether or not Vegas supports more then 2 CPU's. If it does, that would be a fun motherboard to have. If it only supports 2, well then, you sure as hell would have a nice machine for doing all kinds of other things at the same time while making and rendering stuff in Vegas. Sounds fun to be honest, and if i had the money, i wouldnt mind messing around with such a system. Remember though, it would be a pretty noisy machine with all those CPU fans plus all the extra fans needed to help dissipitate heat. And quad motherboards need a big case and a big power supply. If youre really serious about such a system, i would check out the supermicro site....they've been doing quad boards for quite sometime and even have cases and power supplies specificlly for their quad boards. Here is a link to their quad board: http://www.supermicro.com/PRODUCT/MotherBoards/GC_HE/P4QH6.htm
and as im sure you know, these boards use Xeon Processors and they are not cheap. 4 2.0 Xeon's and that motherboard would set you back at least $2200
i personally think the idea is overkill, but from a geek standpoint, sounds like a lot of fun! if you decide to do it, report back and let us know how it goes!
Somehow this reminds me of a guy here at work who loves to tinker with overlocking. He built a water cooling system for his Pentium 90 system and was able to overclock it to about 150MHz. By the time he had it operational it had cost him over $4500 and 200MHz systems were available for under $1500. So next he got a 300MHz system and built a water cooler for that and was able to overclock to 550MHz. Of course, that cost him another $4000 or so and by the time he had it working 800MHz systems were shipping.
I guess my point is that sometimes it's better to just make do with what you've got for a while if it's at least doing the job for you. Probably there will be a 2 CPU system available soon that will run circles around the current 4 CPU system