It's wise to preview reception before you invest a lot of money in a screen. Note that a digital TV station on an older analog set looks like there's no signal there at all--just snow.
If you can clearly see Mt. Soledad, Mt. San Miguel, and Mt. San Antonio (Tijuana) from your roof, you can use just about any UHF antenna with a lead-in to your old analog TV set. If you live on a mesa and getting good reception indoors now, you could try a Radio Shack UHF dual bow-tie, #15-623.
If you don't have an antenna but are ready to invest, Western Radio Electronics promised me they would be stocking some nice outdoor UHF-only antennas by Winegard.
What you need to achieve is reception of a sampling of UHF stations from each of the three sites. A map you print out from http://www.antennaweb.org
will help you figure out the orientation to each site.
I favor trying to receive K35DG (UCSD-TV 35) from Mt. Soledad. It's somewhat weak, but if you receive even a snowy whisper of a signal, you'll probably have enough for decent digital reception on channels 25 and 55, the KGTV-DT-ABC and KFMB-DT-CBS services.
From Mt. San Miguel, you should receive a strong signal from KNSD 39. Reception of KNSD-DT-NBC 40 is a challenge to digital tuners because it is so much weaker than the adjacent channel 39, but I have reports that some tuners are coping with this. With a strong channel 39 signal, you should receive KSWB-DT-WB 19 and KPBS-DT-PBS 30 as well.
From Tijuana, look for signals on channels 21 and/or 27. This should assure reception of XETV-TD-FOX 23.
Be sure to use a high quality RG-6/U coaxial cable. You'll probably have to have a rotator as well, darn it.
Check out my web page for more tips: http://www.kfmb.com/hdtv.php