If you are going fixed - invest in proper rear wheel first, with threading for cog and lockring.
Measure your OLD, buy appropriate wheel (many companies will re-space wheel for you like velomine or wabi cycles)
Typical chainline on older road bikes would be around 40 to 42 mm
Do you have your current crank? If yes - measure the chainline at the crank (http://www.sheldonbrown.com/chainline.html) and once crank is off - measure how long is your axle. Unless you will be buying track/fg specific crank - use the same BB for new crank. There are several ways to adjust chainline - by moving ring inside or outside, by using spacers between bb cup and bb shell, etc. Make friends with your LBS - good shop will help you with bb length and may even let you try several.

Before you do any of the above - spend 30 min and read this http://www.sheldonbrown.com/fixed-conversion.html You will understand what you need better and will likely save some money by not doing noob mistakes. And of course visit singlespeed subforum here (or on mtbr - great bunch of FGSS people there).

And if you really serious about FG and want the best experience - good chainring, cog and chain should be high on your "to buy" list. Sometimes (quite often actually) it is really long process with no guaranteed results to get even chain tension on regular road c-rings and cheap cogs.
FG is addictive so pick your poison in advance - 1/8 or 3/32 chain, cog and c-ring (will hekp you later to easily swap between bikes :-))