GPU acceleration, when does it work? does it? & on what formatts, MAYBE NOT ALL?

Hello all! I have been working with Vegas for 8 years now, editing in HD has been difficult, to say the least.

Im on windows 7 64 bit
I have an intel i7-2600k CPU @3.40 GHz
16.0 GB of ram
& my operating system is on a solid state kick ass FAST drive

a week ago I had an old Radeon 4670 Graphic card, with no GPU acceleration on it, after reading sony's recommendations on what graphic cards to get to utilize GPU because of HOW MUCH FASTER it is, I said LET"S DO THIS! I Bought at their recommendation a NVIDIA GTX 570 put it into my system, updated the drivers, went into Vegas into the video preference tab & engaged the GPU & to my surprise the system is STUTTERING like CRAZY!! 24 hours later I could not improve the performance AT ALL, the following day I went out & bought a second card, a Radeon HD 6950 with 2 gigs of ram, it did not help AT ALL, 3rd day I went out & bought a new FAST solid state drive & put windows 7 64 bit on it, uploaded ALL my programs to this new drive, now the computer is hauling, right? maybe so but not in Vegas, it has done NOTHING. Stutter, Stutter & more STUTTER!!
At this point I just NEED to get back to work, I'm desperate now!! am I going to have to put back a 3 year old card into my system again? that has no GPU? is there ANY hope for GPU acceleration? & maybe GPU only works with specific codecs? I am working with AVCHD straight from my camera .mts files. can this be the problem with GPU ? is it my codec thats killing GPU acceleration? why did my old card work better? with no GPU & please dont get me wrong, my old card barely worked, 1 track only with no effects on in preview mode MIGHT run decent, that's the whole reason why I went with what Sony Claimed to be SO MUCH BETTER $720.00 later my system is running worse now then when I tried to switch over to GPU acceleration
WHAT AM I MISSING!!
Any suggestions