I know this thread is old at this point but while I am sitting here waiting for a project to render on Vegas 12 I figured I would chime in in the odd case anyone else has the same question.

AS far as the stuttering from the old Vaio, I'm sure along with everyone else, I would have trouble without seeing the actual stuttering myself. Also, back then it may have been a playback problem with whatever media player and hardware you were using to view the final render. I can say though that most times when I have rendering problems, a hard drive that is not fast was the problem. This is especially true on older hard drives when you are writing to the same drive that your project files are being read from. I used to work for both Circuit City and then Best Buy in sales and then Geek Squad. I am very familiar with the Vaio computers from that era and wouldn't have ever recomended them. They were pretty much form over function. Even spec for spec, they somehow managed to lose to other machines.

Pretty much all modern computer systems will edit AVHD without issue. Some will just do it quicker or have a more fluid preview while editing. Just for comp[arison purposes and to address the question of the GPU: At work I edit on an I7-930 equiped desktop overclocked to 3.4Ghz, 24GB of ram and no GPU acceleration. At home, I edit on an I7-930 equiped laptop running at a stock 2.8 Ghz with 16GB RAM and an Nvidia GTX560M GPU. The GPU acceleration in Vegas 12 comes in to play for several plugins and when rendering out to AVCHD with the Mainconcept encoder. I believe it also helps in many background tasks such as crossfades and video format conforming etc. while editing.

So, the end result in my experience is that I have a very similar editing experience on my laptop at home with the GPU acceleration even though the CPU is at least 800Mhz slower. To specifically adress the question about which GPU I will say that with the mobile version of the NVIDIA GTX560, I have never gone above 60% GPU usage and that was only when rendering AVCHD with alot of GPU accellerated effects. As of Vegas 12 and only with Vegas 12 in mind, I would say to not spend for the more powerfull GPU if it means the money could be used elswhere. That is not to say there may not be other benefits of the more powerfull GPU in other applications.