The only program that I really top out my processor with (using 10.5.7) is Ableton Live 8.04. Some users have reported massive performance increases with OS X 10.6. However, the Ableton website says:
Live is a 32-bit application. As of Mac OS X 10.4, the Mac version of Live can make use of up to 4 GB of RAM. This still holds true for 10.6.
Does this mean that any performance increase will be minimal, or is that not a logical conclusion?