On one hand, it's far too early to worry about how much Windows 7 you should add into your small business. On the other hand, ComputerWorld's Windows 7: Four Reasons to Upgrade, Four Reasons to Stay Away article does a better job than most in laying out your probable pain points in an upgrade.
My recommendation is to wait until about April to consider whether to upgrade to Windows 7. New systems you buy, especially laptops, will have Windows 7 installed starting in late October, if the schedule doesn't slip. You'll get some Windows 7 if you buy new systems, and that's OK. That's actually a good way to test the new operating system without starting a formal test procedure. Sprinkle a couple around and see if people do more or less work with Windows 7 compared to XP.
After coworkers see Windows 7, and after most of the bugs are fixed in the initial release, you can consider whether to upgrade the majority of your systems or not. If you have the money to replace all the hardware you bought five or six years ago all full of XP, then upgrading will be much easier as you just replace old computers running XP with new computers running Windows 7. Upgrading existing hardware from XP to 7 will be much more trouble.
How to decide? In the absence of any real push to change, such as supporting new critical software that must run on Windows 7, try my Coin Flip Gut Check method. There are two steps to this method, and it will steer you right every time.
Take a decision, such as upgrading. Heads you upgrade, tails you stay with XP. Flip the coin.
The important part is not whether the coin is heads or tails, it's how you feel about the decision. No matter how the coin flip turns out, focus on your immediate reaction when you see the result. If you feel a warm glow of relief, then accept the coin flip. If you feel a tightness in your gut and a frown on your face, choose the other option.
After the relative failure of Vista in the marketplace, we can no longer assume Windows 7 will take over no matter what. Microsoft blinked and kept XP going far longer than they said because of Vista's shortcomings and market rejection. But I'm afraid Windows 7 will be forced on us no matter what. The question is whether you jump in and spend time and money changing early, or you wait and let hardware replacements do the upgrading for you.