I've just bought a Sony T500 camera and took some footage on 16x9 720p HD mode. The clips play fine in Nero or Meda player, but when I load them into Vegas Pro 8, the video is very choppy. I have 3gig of ram and a new ATI Radeon HD 4670 video card. Why is this happening please?
Probably you mean that the video is choppy in the preview window?
If yes, there have been many, many posts here about this (search for "preview") - and the fact is that Vegas is not just playing the videos but has all the overhead of the NLE to put the video through before it is shown in the preview window.
Previewing is tough to the PC. Even on my machine with 2 new quad Xeons it is choppy from time to time. The bottleneck is the CPU power. Even the fastest graphic card wil not help much here.
It helps to preview in a quality lower than "Good" i.e. "Preview" and you should NOT check the "Scale video to fit preview window".
If you feel you are finished with that video in the track, then you can PRERENDER IT. Select it and press SHIFT + M. This will create a video file in your pre-renders folder (specified in options) and you'll notice at the top of your tracks a new black bar is created (letting you know that the event under it is pre-rendered)
When you playback now, Vegas will use the pre-render and it should be smooth because it's just playing the video rather than using the footage in preview, where it may have to drop frames.
Note, if you modify the event after a pre-render, the black bar will disappear again and you will have to pre-render it again.