Yeah, this depends on the environment totally. But if you only have one STU defined, all backups will have to go to that STU. If you have it defined to use both drives, and have multiplexing enabled, then.... well, assuming your policies allow it, both drives would "fill up" with as many streams as the STU allows. But how fast can your media server suck in data? There are so many variables in this equation it's too hard to say what is best without spending a bit of time looking at the overall setup.

Again, it depends on the environment. Is the media server actually capable of pushing a drive up to these speeds? If so, then it depends on how fast your clients can send data to the media server. I have found most clients, even on Gigabit ethernet, can't do much better than about 50 MB/sec. The place I currently work for uses IBM 3592, which are theoretically capable of something like 240 MB/sec. I think we have multiplexing set at about 6 per drive, but then... we only have 2 Gb/sec network bandwith into the media server, so are never going to get the drives up to their maximum rate. Luckily the 3592 drives are pretty good at throttling themselves to avoid "shoe shining".

On the other hand, I have SAN Media Servers (Enterprise Clients, in the newer terminology), that can really pump those drives, so multiplexing on their STUs is set to 1, and the policies don't multistream, and just pump one big chunk of data at the tape drive at very impressive speeds.

Answering the 2 questions above is very difficult without spending some time analysing the environment. Being able to do that is what makes you a decent "NBU expert". It's a bit like "how long is a piece of string"? It depends on where it starts, where it ends, and how many loops and knots there are along the way.

The following question is a lot easier to answer :

Don't use VSP. It annoys me that the NBU Windows client install still installs VSP by default. Don't do that. Use VSS. If you still have clients running Win2K ... well, my practice is if the server is unimportant enough that it can run an out of date, unsupported OS, it's unimportant enough that I can turn off that "backup open files" option entirely. VSP is a pain in the posterior, and I get the impression that Symantec have given up on it in favour of Microsoft's VSS, which completely makes sense, so why should I have to deal with it? Say "NO!" to VSP!
 
Cheers
Dean