Do you think I should make a move on this girl?

I'm a post grad student and I share a flat with an undergrad female student doing the same subject (Physics) as me. She is really smart, sweet and totally hot though she doesn't seem to know it. She walks around the flat in her pj's and sometimes on cold nights she gets into bed with me while we talk, nothing happens though other than all the blood draining from my brain if you know what I mean. It's driving me crazy I want her so bad. I am 25 and she is 20. I am not a good looking guy and have not had a girlfriend for 5 years now and am about to explode with lust for this girl, but she's my best friend as well and I don't want to upset her or do the wrong thing. What do I do?