I can second the idea of cramming as much RAM as possible into the MAC - We run 4GB in ours, and it made a huge performance difference over the 1GB we were running previously. Also means I can have everything open at once that I want to use - helps a lot with workflow.
I couldn't live without Shake and its scripting, I can write something in a minute to do things that would be tedious or impossible to do manually in other programs - but it is an acquired taste for sure, and coming from a programming background doesn't hurt either.

As for the graphics card, it helps if you are running at obscene resolutions, but we mainly see the difference in Maya more than anything else.