i am currently using an Asus eee pc 900HD with an upgraded 2GB of ram plus a intel wifilink 5100 wireless n card, which needed an extra antenna in the lcd (easy to do). The only downside to this little machine is its pile of crap CPU, celeron m dothan @ 990 when its overclocked, which is great for somethings but not for others.
However I would reccomend the Asus eee PC 1000HE, which has an intel atom chip. My buddy has got one and I wish i would have bought something with the same CPU because overall, anything is better than a celeron m in todays market, and the Atom meets mobile needs. Both of us run Ubuntu 9.10 (karmic), XP, and Windows 7 on our little hacktops. If you do plan to run Windows 7 however, get an eee pc 1000 or above because they have the intel i945 chipset vs. my pile of i915 graphics, aka no aero support for me (no hardware scheduler) :P
OH BTW he also gets in the vicinity of 6 hours on his battery with his 1000, but on my 900 i only get somwhere around 3.