I started shopping for a netbook PC as a gift and I quickly realized that the price and processor models vary quite a bit without providing any useful information. So here's a quick list of the common Intel netbook processors and their specs:
Atom N270 - 1,6Ghz, single core, 512k cache
Atom N280 - 1.66Ghz, single core, 512k cache
Atom N450 - 1.66Ghz, single core, 512k cache
Atom N455 - 1.66Ghz, single core, 512k cache
Atom N475 - 1.83Ghz, single core, 512k cache
Atom N550 - 1.5Ghz, dual core, 1024k cache
So the simple quick analysis is, the higher the number, the quicker the processor. Even though the 280, 450, & 455 look identical, the underlying chip sets do get progressively better and the higher models with perform slightly better (although I suspect you won't notice).
I would recommend paying a little more if you can move up to an N475 or an N550. If those aren't available I would make my purchasing decision based on other factors (size, price, hard drive size, etc).
The N550 is the best choice as its dual core, but if performance is your main concern I'd suggest looking at either a Core2 Duo processor or a Core i3 which are both available in Ultra Lower Power versions in netbooks. Either one at 1.3Ghz will greatly outperform any of the above mentioned atoms processors.
I personally have an Asus 1810T with a 1.3Ghz Core2 Duo and its more than capable of handling all of my work needs when I travel.
You can checkout Intel's master Atom processor list here: