Yep, that's all you'll need. You just install Office for Mac, and it does everything that Office does in Windows. If you have any Greek letters in a document and switch between a Mac and a PC they usually turn into boxes or some such (don't ask why I know this), but other than that you shouldn't have any problem opening your existing Windows Office documents on your Mac, or opening your new Mac ones on a Windows computer.
And of course, keep in mind that if you find that you do need Windows XP for some reason you can always install it later.
Oh, and MacBooks have a very cool feature called an accelerometer. The purpose is so that if you drop your computer, it knows it's falling and can shut the hard-drive down to protect your data, but a side effect of that is that your computer knows when it's being tilted, allowing it to do such very cool and completely non-productive things as make light-saber sound effects in response to motion. So you'll be squandering the utterly useless coolness of the MacBook if you don't download MacSaber, and a few other such programs
