Neil,
For a 24" x 24" machine, you might consider going with regular ACME screws and a linear encoder system -- this way you're directly measuring your linear position rather than relying on accurate translation of rotational movement to linear motion. US Digital sells inexpensive mylar encoders you could use, and fairly inexpensive read heads. I wouldn't recommend this for a router due to dust, but for a pick and place system, it should work just fine.
Edit: Link to US Digital site.
http://www.usdigital.com/products/en...al/linear/lin/
Ahren
www.cncrouterparts.com