I have a project that has 84 tracks of audio that is about 2.5 hours long.  I am still running on XP (I really hate OS upgrades, and XP to Win 7 is especially egregious...) with 4 GB of RAM on a Core I5 (quad core).  I've set my latency up a bit, but I still have a lot of chugging going on, even with relatively few plugins, very little CPU usage, and no virtual instruments or anything.  When I hit the space bar it takes about ten seconds for the transport to engage, and if I reposition the cursor during playback, I get an audio dropout that stops the transport, and lots of "Busy" where there should be waveforms.  Any tips for getting better performance out of my rig?
 
Thanks.
 
P.S. X2 handles this worse than X1 does, so I cut up the big project into smaller ones using X1 then switch to X2.

First thing to do is let all the wave forms finish drawing before you engage the transport. Scroll to the end of the project so that the waves that are "off screen" can draw as well. That will slow you down until they're done. Also you may want to adjust your disk buffer settings in Preferences and always raise your latency when mixing big projects.