Asus 1215b is one of the most popular 12.1 inch netbooks out there. It is powered by an AMD E350 processor that comes integrated with ATi Radeon HD 6310 GPU. Asus has been selling it with either 2GB RAM or 4GB RAM modules and 250GB / 320GB hard disk. You can easily upgrade it to 500GB / 1TB HDD and up to 8GB DDR3 RAM.
Folks at insidemylaptop have posted a step by step guide on how to disassemble the Asus netbook. To get access to the RAM, remove the memory cover from the back and you will find 2x RAM modules.
To get access to the hard disk, you will have to disassemble the whole unit. Remove the six screws circled below. Separate the palm rest by using a soft plastic.
Rest of the guide is available at the link given above. You can read more about this netbook in this Asus 1215B review.