Fortunately for you, iWork '06 and Office 2004 both have trials that come bundled with your new Mac. Office '08 will probably have a trial version before it's released and NeoOffice is free, so you're able to try them all and compare them. If you find that one suits your needs better, then go with that one.
NeoOffice is free, but doesn't look as nice as the other apps, in my opinion.
iWork is pretty cheap but doesn't have the features (or a spreadsheet app in '06 - but the next version will most likely have one)
Office 2004 is a good program but it's more expensive and isn't universal, so it may be a little slower with some tasks.
Office 2008 will be universal, but may be more than you need.
Also, Office 2007 (windows) has and presumably 2008 will have a student/home version which is considerably cheaper.