If it's an Eee 901 you're better off upgrading the 16GB SSD. The 4GB isn't really big enough to install Windows 7 cleanly, and the 16GB SSD is pretty damned slow. The recommended options are usually XP on the 4GB and documents and shizz on the 16GB or upgrading the SSD.
I upgraded my 16GB SSD to a Runcore 64GB SSD which is nice and speedy.
As Mnjul said the 901's (and most other Netbooks) do not support RAID of any form