I'm rather attracted to my best friends partner. The worst thing is she lives thousands of miles away.
Why are you mentioning her location?
I'm thinking I might take the plane and go to her, just to see how things go.
First off, you'd have to find out if she's interested in you. Second, you'd be betraying your best friend for a shot at something so uncertain. Third, don't do it.
I've already decided to go and I am planning to have sex with her.
Your friend comes first, what if this partner of your friend rejects you? Then you will have lost the friend and this partner they had. Your life will be so messed up because of your lack of control as far as sex.