When you buy a netbook there are two choices right now for the OS: Windows XP SP3 and Ubuntu Linux (or some variant). Something to keep in mind is that Microsoft DOES NOT want XP back on mainstream desktop/laptops and is only licensing XP for netbooks temporarily until the release of Windows 7 which has a stripped down "starter edition" that will be used for netbooks.
Part of the licensing agreement for XP right now is that that screen size cannot be larger than 10", memory cannot exceed 1GB, and hard drive size is not supposed to exceed 80GB (although I've already seen plenty with 120GB and 160GB drives). The upcoming Dell Mini 10 is rumored to have the 1GB of RAM "built in" so it's not user upgradable beyond Microsoft's maximum specs.
Because of the popularity of the Dell Mini 9 system if you watch the website slickdeals.net you will find that once every two weeks or so Dell will drop a 15-20% discount code to use on their outlet site for refurbished, open box, and scratch/dent models bringing the price down well below $200. Also, if you want to buy one new, they have the option of configuring with Ubuntu Linux instead of XP so you can order one brand new with a 64GB SSD and a memory upgrade for around or under $400 so you have room to multi-boot several operating systems while on the go.