Thread hijack, since it's not worth its own thread. What would you guys do in this situation:
I have a VERY attractive friend whom I've known since freshman year of college (12 years). We've lost touch for longish periods of time here and there, but she's always flirted with me (in fact, she said that the reason she first started talking to me was because she thought I was cute). She's always had a BF whenever we've actively been communicating (emails/calls or hanging out).
Now, she's been with her current BF for about 2 years and is planning on moving to Cali with him since he'll be attending Berkeley grad school there. Thing is, I've gotten pretty close with her over the past couple of months, and it turns out that I completely misjudged her as a person. All these years I viewed her as a sophisticate -- very cultured, refined, and slightly aloof and impassive (though she was always pleasant). Now, while I have no problem with that type of girl, it's not my cup of tea; I tend to prefer more down to earth girls who can watch a movie and dig into a bowl of ice cream with me in addition to doing other stuff like visiting museums etc. I also love girls who have a good sense of humor. Long story short, it turns out after hanging out with her a couple of times recently that she's actually very sentimental and does have a good sense of humor (the determinant of which is whether or not she laughs at my bad jokes

).
So I was pretty blown away by this. Here's this girl who I'm INSANELY attracted to physically, and it turns out that she's really sweet underneath it all. This is pretty stunning because I'm generally a
very good judge of character/personality. Now I find myself growing closer and closer to her. She's forever flirting with me, and flat out tells me how attractive I am, what a wonderful smile and sense of humor I have etc. I flirt with her too, but try to do it more subtly, since I take moral issue with blatantly hitting on another dude's GF.
My question is this: given the fact that I see her in a completely different light, and feel that there might be romantic potential there, is it proper for me to pursue it? I mean, I
do realize it's improper, but on a scale of unethicality, how wrong is it to pursue a girl who isn't engaged? (Being engaged would be a clear "off limits" sign for me.) Another issue I have is that I'm not head over heels in love with her or anything, in which case I might be inclined to go the "all's fair in love and war" route; I simply think that there's romantic potential there and would like to date her. But I can't bring myself to possibly ruin a dude's life just because something
might work out between us, since he loves her and is (presumably) prepared to go the distance with her. After all, what if I threw a wrench in their relationship and then decided down the road that she wasn't right for me -- now she's shit outta luck, and that would suck. I'm generally not a selfish person and do not endorse selfishness (as anyone who's been subjected to my moralizing can attest to

); in fact, I've basically never acted selfishly to anyone outside of my immediate family...but I'm having a hard time with this.
She's staying with her dad now in another state until she moves to Cali (her BF was on a visa that expired, so he can't get back into the US until the Summer). I figure if she comes to NY again to visit her family that we can spend a day in the city doing semi-romantic things (dinner, maybe an off-Broadway show and a nighttime cruise), and I'll be able to gauge her reaction to that as well as to my flirting. I think it'll just get to a point where I say "if things ever don't work out with you and X, I'd like to date you." We're both old enough, and mature enough, where saying that wouldn't be a total fail (and, as I said, she definitely sends enough signals to suggest it isn't one-sided). Then again, I might just go to kiss her at the right moment and see how she reacts.
I'm very conflicted. Mostly because I've always done the right thing in my life (yeah, laugh it up, but I try my best to be a decent guy). Yes, there are many other girls, and I date fairly regularly, but now I'm kinda smitten.
I also realize that this is partly (okay, mostly) a thinly veiled attempt to have others help me rationalize my selfishness and push me past my self-imposed ethical boundaries.

I've just never been - or wanted to be - "that guy" (i.e., a dick), but I'm very tempted to be that guy in this case. HALP!

As usual for my posts, there isn't an abridged tl;dr version. Sorry.
