The best fixie-specific upgrades you can make are a rounder chainring and sprocket, which will allow you to run a tight chain without slack spots. Some people might go for a nice NJS chain but I doubt there is much of a performance or weight advantage compared to any reasonable 1/8" chain (if you are running 1/8"). Hubs and cranks have no real effect on performance either.
You can obviously look at bars, stem, saddle, seatpost, tyres and all the usual stuff.
I'm assuming that you already have the chainline perfect. Check it with a long steel rule (use the edge of the rule, NOT the flat!). It needs ti be within 1mm. You can get away with quite a bit more with a bushingless chain, but knowing it's not right will eat away at your soul.