Office 2003 Standard should so you just fine, assuming all you need is Word, Excel, PowerPoint and Outlook. Office XP Standard gets you the same suite though it's a revision earlier. If you're looking for the latest and greatest, then it's Office 2007 Standard, though you'll be shelling out some serious cash for this one.

If you're feeling a bit adventurous, you might give Open Office a try. You can't beat the price.