The Catalog Builder (in beta stage) might be of help:
http://autocad.autodesk.com/?nd=plantcatalogbuilder
I would suggest, taking an exisiting catalog - SAVE IT under a different name, then delete all parts.
create a part ( I created pipe, 45 elbows and 90 elbows to start )
export it to excel <file> <export to catalog builder>
edit the parameters <in excel> ( you have to unprotect the worksheet usually )
run the catalog builder
build the catalog
and test it... and repeat editing the parameters untill you understand how they work ( for the most part )
This is basically what I did untill I got a rough understanding on how to build parametric parts.
it seems to work fairly well for imperial parts, it totally bugged out trying to make metric parts.