Quick RAM question in regards to Sib 6 -- if I have a machine running 64 bit Windows, and 8 GB of RAM -- but runn Sib 6 (which is only 32bit), am I correct in assuming at most it can only use 4GB of RAM, and that to fully utilize my available memory I should probably upgrade to Sib 7 since it can run 64 bit?

Correct - at most it can only utilize 4GB of RAM. It is likely that for practical purposes it can actually only utilize 2GB of RAM.

One question that is relevant is whether Kontakt functions as a separate application from Sibelius (meaning it would have its own separate 2GB limit) or whether it shares the 2GB limit with Sibelius. Perhaps Jim can answer this. It's important to realize that in a 64-bit environment, even though 32-bit applications are subject to a 2GB or 4GB accessible memory limit, a 64-bit OS can still allow each application to access different portions of memory so that the 2GB chunks aren't shared with each other.

Most importantly, don't upgrade just because of this issue. Upgrade if you think you are having a problem that would be solved by Sibelius being able to access more memory. I have run Sibelius 6 with full marching band scores on a 32-bit OS with a dual-core CPU and 4GB of RAM. It performs almost identically to a newer system with a 64-bit OS, dual-core CPU, and 8GB of RAM. On the older system I had the occasional playback hiccup only.