Indi & Giga both give excellent advice on this subject. You really should follow the Golden Rule in this triangle. If the girl who likes you says anything, remember when responding how you'd like the girl you like to respond if she were to let you down.
Now you said something that caught my eye...
Originally Posted by
Spidy
I'd love to tell her i love her, but i don't want to upset her. And i'm kinda afraid that when she turns me down it will have negative effects on our relationship (as friends)
You said the above statement seemingly with the expectation of being turned down. Go with your gut. If you really feel that she won't reciprocate your feelings, then don't even bother. It's not worth risking your friendship.
I went through a very similar situation with a guy who
used to be my best friend. He professed his feelings, I told him I wasn't ready, he got jealous when I dated other guys, and things haven't been the same since. You can find a more detailed depiction of what happened in the following thread:
I've fallen in love with my best friend.
I feel really strongly about this particular subject because I really miss my friend and I would hate for anyone else's friendship to disintrigrate when some first-hand experience-based advice could have helped prevent it.
Good luck Spidy