Wait until 2008 comes out. There are some important updates if you use Excel if Office 2007 is anything to go by. Not least the removal of the 65000 rows limit, so that larger spreadsheets are possible, quite apart from the predicted speed boost due to Intel support.
There is a new file format, but you can always save down a version, and Microsoft have released the converter
for .docx files already, for Office 2004 and v.X users, although still in beta format.
Another plus is that from Office 2007, the Student and Teacher edition becomes a Home and Student edition, so no edu IDs are required. Again, this is based on the Mac version following what has happened with the Win version.
It will be interesting to see what happens with the Office 2008 for Mac licence, since the Student and Teacher edition 2004 was identical with the full price licence in terms of program content.