I think that the best method is to use a cassette with spacers rather than a freewheel - it's cheaper and you have a better selection of gears (depending on what cassette you have). That cross bike setup sounds awesome. And I find that I lust after road bikes and fixed/track stuff equally. I like hitting a skid, but I also like hitting a shift on an sti shifter. It's all biking - different bikes are better for different purposes. I think that fixies are the optimal bike for fun city biking.