Let's see if I can at least take a stab at these...
Whether VI will take advantage of dual procs or dual core systems depends on two things: if the VIs are multithreaded, and/or if the host you're running them from is multithreaded. For instance, Cakewalk's Sonar sequencer will split hosted VST modules (from what I understand) among multiple threads, and so it will spread a CPU load among multiple processors. I've seen this on my own dual-core, dual processor system. If the VI instruments are themselves multi-threaded and the processing load is balanced properly, then it might balance CPU load even if the host does not. You'll have to wait for a VSL guy to answer definitely, though.
Oh, and *any* two applications that run simultaneously on a single system will take advantage of dual processors or dual core processors, because there are actually more than one processes running. When someone asks of an application if it takes advantage of multi-processors, it's implied to mean "running by itself, will you see a performance increase with multi processors or multi-core". This is basically asking if a program is "multi-threaded", meaning it's written specifically to operate parallel tasks, and can thus be easily split up among different processors by the operating system. So, yes, a dual core or dual proc system will indeed split tasks between Gigastudio and the VIs, if the VIs are launched from a different application.
FYI, multiple processors can give a theoretical ~95% CPU performance boost over one processor. A single dual core processor can give an optimal boost of ~30% CPU performance over a single core in general.
The memory issues is likely a bit trickier. I'm not certain you'll be able to hit the absolute ceiling like that. If VI uses "standard" application-level memory allocation instead of driver-level allocation, then theoretically, you could get much closer to that upper limit. Again, I can't tell you since I didn't program the stuff. Some supposition on my part...
I have no idea if splitting up the instruments on different drives will help performance. All I know is that it's preferable to keep the instruments on a different physical drive than the system drive. Beyond this, I don't know if there's a benefit.
AMD vs. Intel - matter of preference. I tend to stick with Intel purely for compatiblity reasons.
Gigastudio is not supported on 64-bit operating systems. Stick with 32-bit for now.
Note: let me know if I got anything wrong here... [[;)]]