my suggestion is to go to the single speed /fixed gear forum and look at some of the bike direct type bikes they like. For me (I ride about 2000 miles a year on a steel bianchi) my "run to the store" bike is this one http://www.bikesdirect.com/products/dawes/sst.htm It seems to be a lot of bike for $330 all in.

It looks good in flat black but I did have to get a longer stem to make it comfortable (I am 5'10 and bought a 56) and I geared it down a bit with a 19 tooth fixed cog. (the 46 crank -16 freewheel was a little too tough for me) You are way younger than I am so you might like it fine as is. If not you can buy cogs and such as your budget permits after buying the bike.

Anyway go to the single/fixed forum, see what they like, and use the search forum to find out more about any that appeal to you. good luck