Most FS2004 aircraft work in FSX, but might be missing sounds, animations, gauges, etc, because they are incompatible with FSX. I personally watch the FSX updates for new A/C, and there is no T-6 yet. If you have a computer that can run FSX well, you can try the T-6 you found. I got FSX and enjoy flying in it, but i just buoght FS2004 so i can acess more aircraft, have a near lagless experiance, and fly with friends. It depends what you want. Most people will say "Get 2004 until new new stuff comes out" Others say get X and just try and cope with it. I dont want to lead you in any direction, just give you info, but what i did was i got 2004, even though i have FSX. I would get FS2004 and use that untill FSX is a bit less $ and Direct X 10 is out and other things. If you have a super computer that could run FSX, go ahead and get X, and try and find and use a T-6 from 2004. There are alot of addon sites out there, one is bound to have a T-6 panel and sounds compatible with FSX. I woundn't do anything until some more Knowledgeable simmers have posted here, as my knowladge is very limited, i got everything i just said off of other things people have said. But like i said, it what you want to do. most people here still use FS2004.
I hope that jumble of words helped in some way, lol

Lt. Air Force