A Titan with the Big Tow package should not have a problem with a TT that size. Titans are capable of towing up to the 9000 lb ballpark. However, a 6500 lb TT would be towards the upper limit of what I would tow with a 1/2 ton truck.
Now, if you are looking used, I would look more towards to 06-07 models. The 04-05s had too many problems with the rear ends and front brakes. You will definitely want the Big Tow package. It includes lower gearing, hitch and wiring, extendable towing mirrors, and a transmission temp gauge.
Also, don't rule out an 08/08.5 model. With the $5K rebate it's not uncommon to get deals that are $8K-10K below MSRP. The 08s have a higher GVWR (7200 lbs) and larger front brakes. The long bed models also have a 37 gal gas tank, which comes in handy when towing.
2005 Rockwood 2501
2008 Nissan Titan SE, Crew Cab, Long Bed