I use Logic, but I assume the same principles apply.
I'm using a 2008 aluminum Macbook that I've upgraded to 8gb of RAM. I use a lot of plugins as well. In terms of software instruments, I use stuff mostly from Native Instruments. Several instances of Guitar Rig (4 rhythm guitars, a bass guitar, a lead guitar), several instances of Absynth, Massive, and FM8, EZDrummer, and a few tracks using Logic's built in sampler. Throw in EQ, reverb, filters, various compressors, analyzers, limiters, etc... and, well... yeah... a lot of plugins...
Having a lot of RAM is useful mainly for anything sample based, I've found. Most of my instruments aren't sample based. The synthesizers all use processing power, but don't play back samples. Guitar Rig uses processing power as well. The only plugins that use significant amounts RAM are the sample based one. EZDrummer with Drumkit from Hell loaded uses about 450mb on its own. The various other samples I use take another 500mb or so.
So, find out specifically what your plugins are using. Regular audio within your project will be loaded into RAM as well. In my case, quite a few of my tracks are MIDI, so they take up negligible amounts of RAM.
For me, upgrading to 8gb of RAM made basically no difference in performance. When I had 4gb, I usually had about 500mb free while running Logic. Now with 8gb, I have 2-3gb free at all times (bear in mind the Kernel task uses more RAM if more is available). But, I'm still getting constant overload errors while mixing. I have a good half of my tracks frozen (ie. the individual track is rendered with the plugin and temporarily replaced by an audio file), but I still have constant overload errors. For me, the cpu is most definitely the weak link.
There's a Logic benchmark on the Gearslutz forum that tests how many tracks with a plugin you can play live before you start getting overload errors. The numbers aren't necessarily representative of reality... it's just standardized, so relative differences are quite real. My Macbook maxes out at about 17 tracks running that benchmark. I tried it on a few Mackbook Pros at the Apple store. The i7 equipped 15" Pro maxed out at 73 tracks. The i5 ones made it to about 50 tracks. The best of the dual core Macs seem to be able to hit maybe 30 or so. I can't speak for Pro Tools, but Logic benefits heavily from having as many processing threads as possible. So, a quad core i7 with hyper threading (8 total threads) is the ideal configuration. Something I intend to upgrade to once the Ivy Bridge Macbook Pros are released.
Hope that helps
