I'd suggest selling your single speed bike as-is through Craiglist (if you were in Philly, you could easily get $300 for that bike) and buying a geared bike as a replacement. You'll end up with something quite nice even if you don't put all of the money made on your single speed sale towards the geared bike. Unless you have some sort of emotional connection to that frame, selling it will be cheaper than converting it. If you have a mechanic friend, you could always buy a geared bike and swap the necessary components over to your current frame then sell the remains of the geared bike.