ISBoxer now automatically selects all cores for RIFT when you make your team with the Quick Setup Wizard (as of any new download of ISBoxer, just did a ninja update).

If you play WoW, then the same as what you do for WoW is probably fine. If your 8 cores are from an i7, see http://isboxer.com/forum/viewtopic.php?f=30&t=1090 for recommended settings