No harder than upgrading a laptop. One screw underneath a sticker on the base then run you fingernail along the join all round to pop it open. Memory slots are right there on top for easy access.
Yes, 2.5" sata, it's sitting right next to the memory slot when you open so it looked pretty easy to change (although I didn't). No idea if it is good enough to play HD in windows - you might want to check that. If you want to install Linux+XBMC there is a good guide
here - I'm a linux noob and managed it no probs.