As mentioned by Paul R, it's worth checking Activity Monitor to see how much RAM is actually in use and whether or not more RAM will help. Premiere CS4 is not 64-bit on Mac OS X, Premiere itself can not use more than 4GB of RAM on it's own.
The biggest advantage of having more RAM is that it allows you to switch between programs without requiring to page in or out memory from the hard drive. The hard drive is often times the bottleneck when it comes to speed. (eg. Your comment to SamB's answer re: Mac OS X slow at start up, logging back in, etc. is all the hard drive speed and not the amount of RAM at that point).
Mac OS X loves RAM and will use up as much as possible before touching the hard drive for virtual memory. I'd recommend upping the RAM as the speed difference in switching applications and not having to worry as much about disk swapping makes life much easier. How many other programs are you running at the same time? If you're running more than 2 or 3 you should notice a difference. But to determine a significant improvement it's difficult without knowing the amount of RAM Premiere is actually using (eg. Final Cut Pro isn't a RAM hog - it's constrained by other factors first) and whether or not the competition it has for RAM is actually causing a performance penalty.
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This blog post pertains to After Effects, but it should give you some ideas... http://blogs.adobe.com/toddkopriva/2009/12/performance-tip-dont-starve-yo.html
– Joe Internet – 2010-04-01T22:57:14.080@Joe Thanks for an informative link. +1. – Moshe – 2010-04-02T00:50:37.133