you can buy a flip flop (or, as i prefer to call them with my endless amounts of class, duplex-style hubs) and build it to a rim, or buy one premade (i suggest www.irocycle.com - smooth hubs, solid rims, exceptional wheelbuild), to have the option of fixed and freewheel. but honestly, i find fixed (with a bit of experience) to be really helpful when traffic is tight. i just lean against the pedals and slow down enough to slide between that bus and that cab. getting really precise control over a fix does take time (i've been riding daily for two years and by no means consider myself to be Captain Awesome), so i'd suggest you make the transition and work up to it, especially if you like it. however, if you want to have both options on one bike, go for a duplex style hub.