If you buy the Home and Student Edition of Office 2008 (which retails for $150, usually a bit cheaper on some place like Amazon), you can install it on up to 3 computers. Home and Student Edition has Word, PowerPoint, Excel, and Entourage. It doesn't have support for Exchange servers, and it doesn't have built-in Automator actions. If you don't know what either of those are, or if you just don't care, then Home and Student is probably for you. 🙂
If you need Exchange support or if you really want those Automator actions, then you'll need to get either Standard Edition ($399) or Special Media Edition ($499, which also includes Microsoft Expression Studio). Standard and Special Media can only be installed on one computer.
Regards,
Nadyne.