This thread will show you the kind of statement to get stuff for just one category...
http://forum.x-cart.com/showthread.php?t=44976
I will post you another thread I was just in that showed how to remove those buttons (we did it for items of 0 dollars ... this making them call for price ... so there was no need to do any category stuff, you could just set all items in that category to 0 dollars.
EDIT: here is the other thread with the call for price information:
http://forum.x-cart.com/showthread.php?t=18727