Mac's are technically not supported except by using the BootCamp software (making the machine a dual boot with XP).
I have had students find that using a Mac with Parallels works fine for SAS 9.1.3 and EGuide 4.1 as well as SAS 9.2 and EGuide 4.2. Since Parallels is an XP emulator, there is a small performance hit.
Windows 7 isn't supported yet, but does work. See
http://blogs.sas.com/sasdummy/index.php?/archives/128-SAS-9.2-and-SAS-Enterprise-Guide-4.2-on-Window...
Spending your money on the fastest disk drive you can get will generally pay off the most for big data. If you are using some of the newer compute-intensive stat tasks (mixed and the Bayesian ones come to mind), then a fast processor and lots of RAM will also help. Generally, I find the processor loafing while SAS is running big data for things like regressions or other deterministic stat tasks.