i had the same situation! i fancied a girl who i joked around with, plus a another girl i was friends with fancied me at the same time.... me and my girlfriend were just like you were with the one you fancied, i sort of asked her out, she sort of said no, then 2 weeks later she said yes... that was january 2004 and we're still together now.
forget the one who fancies you, they'll get over it, and go for it with the one you like. i was'nt sure if they like me or not, but i had a feeling and i knew i wanted them , so i went for it and i am very glad i did.
go for it.