I have rendered several projects to 1080i and 1080 30p with no problem on a I7 with 6gb. with vegas 9a. My problem was burning to dvd or blu ray and found I had to slow the dvd burn to 8x and 2x on the blu ray
I used Vegas multicam with up to 6 sd and hd cams for a music video i just finished. I worked excellent. you can see the video at youtube.com/geimanbros the video is birdland
I do it all of the time. You just need to find the actual clip folder (it seems the cameras generate 3 or 4 folders for each clip) you can tell the actual clip by the size and extension