This may be too broad and sweeping a statement, but its
something I firmly believe in. So here goes.
"All's fair in love and war". Cliched but true. Basically my
take is this, it doesn't really matter if a girl is attached or
not, as long as she isn't married! If you like her, then go for
her. At the end of the day, its the girl that will be making
the choice between her current boyfriend and you. Then again, if
you succeed in a "hostile takeover" as it were, would you be able
to fully trust your new girlfriend? I.e. If she broke up with her
ex to be with a better guy i.e. "you", would she do the same to you
if some other nicer guy came along?
The issue at hand, is that you yourself have a girlfriend.
Which means you have responsibilities to your current
girlfriend. Do yourself a favor and try to
reconcile the differences between you and your girlfriend first.
The last thing you should ever do, is to reciprocate feelings
towards this girl that likes you whilst you yourself still are
attached. If you really feel that your current relationship
doesn't have any long term prospects, let your girlfriend know and
end the relationship. Once you have a clean break then see
how things work out with the other girl. Its much easier on
your conscience and less stressful to have a clean break, instead
of two-timing and trying to hide your activities from the
girls.