Here's the number one thing to do: DON'TSHOOTONHDV!!!!!
HDV's color resolution stinks on ice. It's God-awful. Since having the highest resolution colors possible makes keying easier, HDV is NOT the tool of choice. Know how much fun it is key DV? HDV takes the fun to new heights.
Then, what sort of motion do you want: the live look of interlaced video, or the more film-like motion of progressive-scan? If you like progressive, there's a problem: 1080p cameras (other than HDV, which you do NOT want) are very pricey, so 1080p is probably out.
Some folks claim you absolutely must have progressive-scan, but I've never had a big problem keying interlaced video. If progressive-scan was a necessity, how did people ever get good-looking keys ten years ago, when progressive-scan didn't exist? I seem to recall seeing good keys then.
So what to use? 720? 1080? I think it's a toss-up, but if I had to choose one, I'd go with 1080. More resolution y'know... which could be helpful in a pinch.
Anyway, that's my opinion. But there's one thing that is NOT opinion: for chroma key, avoid HDV like the plague.
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