I've recently been building up an old road bike frame, and I'd like to make it a single speeder. So far, I have pretty good parts on it: a Campy NR crankset, Phil woods BB, Brooks saddle, Campy brakes, etc.. I was just wondering the importance of the rear cog in overall performance. As of right now, I'm planning to use a plain, low end Shimano cog from a cassette. Are there any particular brands or types of cogs that are designed specifically for single speed (not fixed gear) bikes, or should I be fine with the Shimano cog? I've dumped a lot of money on this bike, and would prefer not to spend more, but if it would make a big difference I would shell out for a high end cog. Thanks for the help.