Although the main aim of Snow Leopard is to make it a slimmer, leaner OS, I don’t think you will see much difference in performance on your current system. It will still take x seconds to encode and mp3 for example because that is down to CPU speed.
From all the benchamrks I have seen i7 is up to 30% faster (depending on the task) than the current Core 2 CPUs.
I think new i7 Mac Pros will be along soon-ish as the i7 processors go on sale in November also the Mac Pro is overdue a refresh…
https://www.rhythmac.com/history/macpro
Basically if you can wait, do, but it might be up to 3 months before you see the new ones.