These are pretty much my guidelines on what MacBook to get:
MacBook:
Avoid it. It hasn't been updated in 1.5 years, and it shows its age. The CPU is a generation behind. The ONLY reason I would go with the MacBook would be for the nVidia 320M, which is SLIGHTLY better than the Intel HD 3000. Even then, the Intel HD 3000 outperforms it in most cases, and the nVidia 320M is irrelevant anyway.
MacBook Air 11"/13":
If you need a portable computer, have a small media library, and mostly just browse the Internet, the MacBook Air will be perfect for your needs. On a side note, it's rumoured that the MacBook Air is about to receive an update. If you're interested in the MacBook Air, and if you can wait, wait till the end of the summer to see how the rumours pan out.
MacBook Pro 13" i5:
If you occasionally dip into casual games, have a decent sized media library, and want a good all-round computer with more flexibility than the Air, the MacBook Pro 13" will suit you perfectly. I suggest sticking with the i5 in most cases, as anything that requires the i7 will most likely require a better GPU, etc/
MacBook Pro 15" 2.0GHz:
If you are into casual gaming, casual video editing, or photo editing, the Quad-Core CPU and AMD GPU will be a significant upgrade to the Intel HD 3000. I also suggest going with the Hi-Res Display for the latter two. The extra screen estate makes a big difference. If you can though, I would spend the extra money on...
MacBook Pro 15" 2.2GHz:
A powerhouse machine. The AMD 6750M is another serious upgrade to the AMD 6490M. Great for gaming and video editing. The CPU is also great. I would avoid the 2.3GHz upgrade, as the extra 100MHz (which is made irrelevant with Turbo Boost) and 2MB in cache size is not worth the extra $200. Unless you KNOW it's going to make a difference in what you're doing, it's very unlikely you'll notice a difference in performance. One last thing to take into consideration is the...
MacBook Pro 17" 2.2GHz:
I would also consider going with the 17" MacBook Pro, as it's only an additional $200 for an increased screen estate, additional USB port, and an ExpressCard slot for eSATA. The weight differences is not that bad either.
That's my summary.