Greeley will be an issue, I believe the channels come from several different towers in different locations vs in Denver where everything pretty much comes from Lookout Mtn..
There is also the issue of different transmitting power issues for each station and the fact that some frequencies travel better than others.
I install TVs for a living and what I typically do when a client wants an antenna is I get up on the roof with a basic flat antenna (one that isn't tuned to receive any frequencies more than others) and I use a meter to record the signal strength of each station. You might get 3db on a low VHF channel, 9db on a high VHF channel and then 12db on a UHF channel (this is a generalization because the way things are transmitted even though you tune to channel 7 it is actually broadcast at a different channel number in the UHF range). I then take those numbers and look at the different antenna specs that the manufacturers provide and I would find one that has a high boost on low VHF, a low boost on high VHF and no boost on UHF.. What you want ultimately is an even signal level across all channels as the signal hits your TV. If you put a preamp or amp on an antenna that is not tuned then you are still just going to get a shitty signal on the lower signal stations.
Without the proper equipment you kind of just have to make a guess..
This antenna has been working well for me in the Denver metro, can't say what it would do in Weld County
http://www.winegard.com/flatwave/air
Use this link to see where your towers are and what antenna they recommend based on your location
http://www.winegard.com/selector?q=offair