As a prerequisite it is important to do some research as to what, if any, issues there may be with any 3rd party stuff you are using. I moved to SL several weeks ago on both my Dual Quad and Mac Book Pro, and feel that it is a little snappier than 10.5 was, Logic is working well on both Macs. I did have the problem of not having access to the TC powercore stuff, but since TC released a public beta, these plugins are now back up and running on my system.
You also need to think about how you want to upgrade. The safest, but most time consuming method is to swap drives, pull out your old system drive, put in a fresh one and start installing everything from scratch. THat is easiest on a Mac Pro, more difficult on other macs.
I didn't actually do this this time, rather, I made a super duper clone of my system, upgraded it and ran with it for several weeks, before finally upgrading my original system drive ( I took a final clone of that onto another external HD before taking the plunge in order to have one last "go back" option.
You need to be aware of a couple of options when running the upgrade - it offers you the chance to keep QT Pro 7, which you should definitely do IMO. Snow Leopard's QT 10 is missing some important functions which you may still want to access. Also, if you are using any rosetta based software, you need to check the option to install rosetta when upgrading.
I'm presently running OS 10.5.8. If I use Vienna Ensemble Pro server as a standalone in 64 bit with 64 bit Spectrasconics instruments loaded into it, would there be any benefit? If I upgrade to Snow Leopard would I then see a difference in RAM management?