I just got an HDTV and went through this same process. Here are a few things I learned:
It is true that currently HDTV over the air broadcasts are on UHF because the VHF spectrum is still being used for regular TV. But, once the old TV is turned off, the broadcasters can opt to use their VHF frequencies for their HDTV. Or, they might use or lease them for other uses and keep the HDTV on UHF, depending on the particular market siituation.
So, to be safe, I got a TV antenna for both VHF and UHF (most of them are like this anyway). If you currently have satellite, you may be able to use it to pick up your local VHF channels and avoid the extra monthly charge for them via satellite.
As for range, there is a "cutoff" effect for HDTV that is not present for regular TV. Similar to FM radio, if you are out of range you get NO picture, unlike regular TV that just gets more and more noise ("snowy"). Generally, if the antenna works well for regular TV reception, it will work well for HDTV. However, right now many HDTV broadcasts are not yet full power or using the best antenna, so you could have a reception problem. It depends on your particular market and distance.