If your Office usage is light, you may be able to get away with Open Office or NeoOffice, which are both free. If it's moderate to heavy, MS Office 2008 should do the trick.
But if your Office usage is intense (Excel macros and the like), you'll want to have a Windows virtual machine available, in which to run the real thing. Parallels and VMWare both run Windows quite well in a virtual machine. I use Parallels to run both Windows and Linux (the latter mostly).
And, Go-oo is another version of OpenOffice.org that includes some additional goodies, in particular VBA support (or so they claim; I haven't needed that feature).