Take if from an Engineering PhD student who uses spreadsheet, word processing, and presentation programs all the time: DO NOT use NeoOffice. I don't understand why people keep suggesting this program, besides the fact that it is free and can open documents from MS Office. It is everything that isn't Mac. It is clunky, slow, and not intuitive. Don't get me wrong, I love the fact that the folks at NeoOffice are trying to make a free version of Office for the Mac, but they failed miserably at this task. Instead, purchase iWork for $79. For word processing, use iWork's Pages. It is far more intuitive than Word and much prettier. Replace PowerPoint with iWork's Keynote. It is smoother and more versatile than PowerPoint. As for spreadsheets, I must admit that there aren't many great Apps out there for this function. Just read this article
to get an idea of everything spreadsheet, when it comes to Mac. But I must repeat, DO NOT use NeoOffice. Then again, if you are really cheap (which you probably aren't because you bought a Mac) and you wanna eat up all your RAM when opening large spreadsheets, then try Neo out and feel its wrath... lol. I could go on and on about Neo's problems, but that is not for this thread.
In conclusion, don't be cheap. Just get iWork and one of the spreadsheet options suggested in the article linked above. Don't worry, Apple will come out with a great spreadsheet program very soon.