The highest score possible right now in any category is a 5.9. Theoretically, the score is between 0 and 6.0, although it will be adjusted/augmented via Windows Update in the future.
The only computer I have seen to score a 5.9 overall (ie. in all categories) was a dual processor (two 3.0GHz cores on each CPU) QuadFX machine with 4GB of memory, RAID 0 Raptors, and dual Geforce 8800GTX graphics cards. It was a prototype/demo machine from AMD.
If you are achieving a 4.0 or above, you have a very fast machine. A 5.0+ means you're at the top-end of what's available today. 5.9 would mean you're beyond what's expected to be even available to regular users today.
Your "overall" score is limited to the slowest component in your system. In the case of the MacBook, you'll be limited by the integrated graphics chip, which still should achieve a respectable 3.1 or so. Not bad at all for integrated graphics, which is more than enough to run Aero very cleanly and perfectly fine for anything except high-end gaming.