FYI, Texas Titles boat and outboard motors separately, but registration only on the boat.
A quick search says that boats are not titled in TN at all, so you should just need the TN registration certificate and TPWD forms PWD143/PWD144 need to be signed by the seller. YOu need to complete PWD 504 (pencil tracing the hull and motor IDs.
http://www.tpwd.state.tx.us/faq/fishboat/boat/registration_and_titles/#faq15Give your local office a call if you have some more questions, some are very helpful, others not so much:
http://www.tpwd.state.tx.us/fishboat/boat/owner/office_locations/As Mouth said, trailer is at the tax office and you will need the bill of sale(have it itemized with boat/motor and trailer) and the old registration to get it registered in Texas. Have the trailer serial number handy. Boat trailers("small" trailers in general) are not titled in Texas.
[This message has been edited by Brad06ag (edited 3/8/2012 7:20a).]