Help converting
 

I am about to start buying parts for my conversion to fixed gear, but i need to know which parts i need to buy . I am planning on buying a new velocity rear wheel from IRO, how do i figure out exactly which size hub i will need...... 120, 126 or 130mm? besides a cog, lockring and chain do i honeslty need anything else? Can i recycle my current bottom bracket (which is relativley old) or should i look into purchasing a new one? If i do decide to purchase a new bottom bracket how do i figure out the size i need? MOst i have looked at have 68mm shell size, is that standard for an older road bike? And in terms of spindle length what is best... i have looked at everything from 107-111mm? Your help would be much appreciated. If there is anything else i am neglecting to remember please help me out.
muirfield

This is the general how to:
http://www.sheldonbrown.com/deakins/...onversion.html
This is for measuring your drop-outs:
http://www.sheldonbrown.com/frame-spacing.html
bottom bracket:
http://www.sheldonbrown.com/bbsize.html

I use a 110 spindle lenght and have had no problems getting a straight chainline. My rear drop-outs are spaced at 120mm

I would suggest surfing everything Sheldon Brown has to say on his site. He is a guru for a reason.