Rich,
Lot of this depends what you expect out of HD. Can you get locals in your area with an OTA? Do you want DVR capabilities? type of programming you watch? What will be cost for each option. THese are all things I would consider.
The 811 can provided both OTA and Dish HD. Without OTA, I am not sure if the HD pack is enough. Depends who you are. I have an 811 and I am happy with mine but others are not. Depends on your use case and confidence in getting an OTA signal. If you can get a good OTA signal, feel that you want run into multipathing issues, and you can get the 811 at a good price then it is a good deal.
As for Voom? not sure no personal experience. Personally if I could get HD on my set for around 200 bucks I would do it. But that is me. Opinions on this issue differ and it is personal choice.
Since you already have Dish, Dish would be the less painful process to go through.. I would add that to the equation.
About two years ago, I was faced with the decision to go with a 6000 receiver and get HD now or wait for the upcoming 921 and have HD with DVR capabilities. At the time there was only OTA and HD Discover and HBO HD. It became obvoius to me that since I invested so much on a HD TV LCD 60", that I should have HD as soon as possible. I decided not to wait the 6 months for the 921 to come out and went with the 6000. Turns out that was a good decision for me because the 921 did not come out to over a year later so I would have not had HD for that year.
At the time the 921 came out I was forced with going 921, staying with the 6000, or moving to an 811. I jumped to the 811, sold my 6000 on ebay, and I am not disappointed with my decision. For me having such a big HD TV and no HD is just wrong. ; But that is me. The entry point is much lower that it was a year ago and there is some compelling content out there. Not as much as us HD folks would like, but there is some. At this point it is still a mixed bag, but a bag I am find enjoyable from time to time to make me feel it was the right decision two years back.
Good luck.