I followed the UD3P guide at:
http://leopardsoup.pbworks.com/
Full parts list:
http://secure.newegg.com/WishList/Pu...umber=10429825
As mentioned on the Leopard Soup wiki, the motherboard and graphics card and the parts you most need to check for compatibility, but that's not too hard as InsanelyMac has a forum dedicated to hardware discussion.
Other things you'll want to read before attempting any build:
http://leopardsoup.pbworks.com/survival
http://leopardsoup.pbworks.com/shortcomings
http://leopardsoup.pbworks.com/howstuffworks