From what I understand VOOM may be on shaky ground. They have far fewer subscribers at this point than they anticipated. And while they do offer more HD than any other provider ~at present~, they will soon lose that advantage.
Directv is going to be greatly expanding it's HD content over the next few years. They have had limited satellite space available for HD for some time...and the space they did have they have had to keep reserved for Sunday Ticket HD. This coming weekend is the final one for Sunday Ticket, and it's expected a few more HD channels will be added to their lineup shortly thereafter as they will now have a bit more bandwidth to work with. It's very likely TNT HD will be at the top of the list.
However, the big news is that DTV is due to launch 4 new satellites within the next 3 years, all devoted almost entirely to HD. Two of them will be launched in 2005 - one around April I think, which would make it operational around July or so(?), the other in the fall. The other two sat's go up in 2007. The first satellites will carry mostly local HD channels (CBS, NBC, ABC, FOX, PBS) for all of the largest markets in the US. For the time being you are only able to get the major networks in HD if you 1) live in NYC or LA, or 2) live in a very remote area of the country not local to any major network affiliate (unlikely, only about 10% of the continental US falls into that category), or 3) have waivers from your local affiliates (which are not easy to obtain in many cases). However, by mid-to-late 2005 anyone within the territories of about the 40 largest US markets will not have this problem anymore.
As I understand it, given the high cost of VOOM and their tiny market share, when DTV gets these new birds up it's basically Doom for Voom, as they will not have a competitive advantage in any area over Directv. So you might want to investigate things a bit further before you invest too much money in the equipment. Avsforum is an excellent place to start.