SB.........Single speed is easier. Just take the current 5 speed freewheel off and put a bmx freewheel on it with a 1/8" bmx chain. I might have that stuff I could toss your way. To do it nicely you would need to re-dish the wheel as well. The easiest way to make a fixie out of it would be to buy a pre-made wheel with a cog and lockring and swap the chain. If you want to do it cheaply (which I don't recommend) you could again take off the current freewheel and spin on a track cog and a bottom bracket lockring with some locktite to hold it together. I think you could possibly open yourself up for a liability lawsuit that way though. There can be some issues with chainline as well.........
The differance between a fixie and a single speed (even though technically they are both a single speed) is a fixie doesn't coast. If the wheel moves (forward or backward) the pedals move accordingly. Pm me for more specifics if interested. As you know I've gone this route a few times.

Kelly D