Intel Mac users will like Snow Leopard's smartly designed interface enhancements, and its Exchange support is a must-have (especially with Outlook for Mac on the way). With a ton of technological improvements, Snow Leopard is worth the $29 upgrade fee.
Windows Vista Business is essentially warmed-over Windows XP. If you're currently happy with Windows XP SP2, we see no compelling reason to upgrade. On the other hand, if you need a new computer right now, Windows Vista is stable enough for everyday use.
Despite its many annoyances, XP is a worthwhile upgrade. Unless you need corporate administration tools, XP Home Edition should suffice for most--even for small businesses. Enterprise customers, look to Professional Edition.
For small businesses that need a business in a box and want to stick with Microsoft technology, SBS 2003 is the only game in town.