I use 3/32nd stuff on mine (that's the regular pitch that you get on geared bikes)
I know most track bikes use the 1/8th inch pitch ,but the consensus seems to be that 1/8th is not that much inherently stronger.
In my experience, a lot is made of chainline on fixed bikes with many aiming for perfect chainline and feeling rather chuffed with themselves when they've got it spot on. However, I'm not sure that it's that crucial. I've had a perfect chainline on a fixed and another one that was about 8mm out, and as long as there is a decent tension in the chain, I couldn't tell the difference and never suffered any adverse affects.
I don't know whether anyone’s mentioned this, but there's a nice web site called FixedGear Gallery, where people post pics and a brief description of their fixed steeds:
http://www.fixedgeargallery.com/
There are some nice and some wacky bikes in there
EDIT:
Marco's right about the 53.
It's not a great idea since it doesn't give you many options to change the gear size. Even a 19t sprocket is hard to get hold of and most are 15,16 17.
Best to start of with a 42 or 44 if possible.