Hey Gander,
THX is not a listening mode, it is a set of specifications. So, if your receiver is THX certified, it means the manufacturer has sent if off to THX to evaluate and if it meets their requirements it will get a THX rating. Hence why there are different levels of THX, such as Ultra 2 etc when you go higher end (however don't assume because a product is not THX certified that it is no good, THX certification costs a lot of money, many smaller manufacturers won't bother even though their kit may be well in excess of the THX spec).
Generally if you're watching a DVD, choose the best quality soundtrack on the disc. DTS if the disc has it, otherwise just plain old Dolby Digital.