[MENTION=185471]WhyFi[/MENTION], I really don't know if it matters, and Dan can probably shed light, but I have read that you are better off with odd numbers for gearing on single/fixed bikes. If you add the chainring to the cog, make sure the total is odd. Now I don't know if it only matters on fixed gear, where you are looking to disperse skid patterns, but I have read it in regards to SS'es as well.

Also, I have had excellent luck with Excess freewheels, bought through Universal Cycles. Lester pointed out that sight, and it has definitely fueled my personal cycling schwag addiction.