I recently decided to build up my first real fixed gear (Soma Rush) as well -- here's what i decided:

Spend a bit more on the frame, wheelsets and cranks, etc. I spent about $1200 all told. You can always upgrade components later -- all of them if you choose. I know you can do this with wheels and frames, too, but to me the frame and wheels were the 'soul' of the bike and I decided to spend more there up front and I'm happy I did. I had the wheelsets hand-built and they are awesome.

I've recently saved up a few hundred more so I'm off soon to upgrade my heavy-ish steel seat post with a Thomson Elite and the heavy-ish Salsa stem with either a Ritchy or Thomson as well. This spring I plan on possibly upgrading the BB and also the crank to 75's.

IMO this was a good way to go about it. Then in the future if I decide I need a nicer frame I'll slap the 75's, Thomson, etc. on it and already have great parts and the mid-range components I had previously will be put back on the old bike -- perhaps with some changes to vary things up a bit.