From what I've heard Win 7 is the way to go. Mind you this isn't 'official' word, just people I've 'seen on the street' who say their netbook seems to run much better on Win 7.
Also, keep an eye out for the newer ones. The Ion based ones should be out already and will feature 12" screens with higher res and more power.
Odd... Asus' Eee PC (1201N) with the Ion doesn't appear to be out yet. Last I heard it was slated for late October (over a month overdue now.)
Also, if you have a bit more to spend you can get way more machine for not a lot of sacrifice. As example, the Sony Vaio VPC-CW190
base model is $670, which has a lot more powerful CPU, low cost upgrade to 4 gig, 14" screen, Nvidia G210M base GPU with a G230M upgrade for teh cheap, and 1366x768 res (which the Ion netbooks should have, but on 12" screens). (Actually everyting appears to be out of stock - Blu-ray upgrades were free when stuff was in stock.) Base weight is very close to netbook, though a larger battery (which would be helpful for Blu-ray playback unplugged) is going to increase that.