HD in Vegas

Okay, I hope I explain this correctly:

I shoot (usually) in HD, then capture the footage into Vegas 6. It captures as m2t files. I'm working with a 2.4ghz PC, so I have to convert the m2t file with Cineform - which converts the file to avi.

Now to my question. I want to edit the footage and then transfer my finished product back to HD (HD to DV tape). I'm not understanding the process for some reason. How can I transfer a file from m2t to avi, then go back to m2t on DV tape? I'm getting into cable commercial work, and I want the best visual quality to send to video production studios to transfer to BETA SP.

My second question would be, should I just get a faster computer and avoid the HD to DV tape. If so, what's the least amount of speed I need to effectively edit HD?

I'd aprreciate any help that's available.