To the OP,
If that is the only spare PCI slot in your picture you posted underneath your graphics card i would be very careful as the soundcard will probably block quite a lot of air-flow to your GPU.
I had the same issue crop up when i got my GTX 560 and where i usually had my soundcard was just beneath it. I was lucky though and had another spare PCI slot above and so moved my soundcard to there. All heat issues were solved from there on.
So may be before you go forking out for a soundcard, see if you can borrow one off a friend just to check before you commit. Good luck with it and i hope you get things sorted