It's not a whole lot more powerful than the Atom I don't think. It may be less
powerful, but their design is so different that it's hard to compare. The atom is an in-order processor with hyper-threading...somewhat similar to something you'd find in a video game console. The HT will probably
make it better in multi-threaded applications, which is what new programs are moving toward.
With today's modern websites, I'd recommend
at least a 2Ghz dual core. The Core 2 duo series is, IMO, the best current mobile processor.
On the other hand, if all
you need is portability, and you need a lot
of it, you could just go for a netbook. It'll have a tiny screen and , yes, low processing power, but the 8-9 hours of battery life is more than you'll get from any
conventional laptop. The problem I see with them is that websites constantly need more and more powerful computers to run smoothly, and netbooks will start to fall behind much faster than more powerful laptops will. If you don't care about that, by all means grab something with an Atom. The N450 just came out recently and has better graphics performance than the N270/N280, and 40% less power usage.
The 13" MSI one you listed is basically a netbook with more RAM and bigger screens, like I said, so if you might save money buy just getting a full-blown 12" netbook and upgrading the RAM. The smaller screens will make for better battery life, and will be the only difference. Well, that and the price....$350-$400 for a good one.
I'd go for this, personally:
N450, 10" screen(1024x600), bluetooth, webcam, windows 7, supports 2GB of RAM, and is supposed to hit 14 hours of life.
edit: Just checked, and the atom-powered MSI you linked has the old Z-series one, which uses more battery and lacks hyperthreading. I'd avoid that.