Great question and I think I can help you with what you need.
First, you need to find our if your Rear Projection TV has a build-in HD Tuner (also known as an ATSC Over The Air High Definition Tuner). One way to find out is by buying an HDTV antenna and plugging into the RF/coaxial connection on the back of your TV. Then you would go into the menu of your television and under the "setup" section you would pick the cable settings.
In cable settings - make sure your setting is set to "Air" (and not on "Cable")... this tells your TV that you will be using the standard Over The Air frequency.
Then, there should be an option for "Scan" or "Channel Scan" which will then start to look for over the air channel frequencies.
Once your TV starts to scan for channels it will tell you that it has found X number of Analog channels and X number of digital channels. If it says digital channels, it means you have a built in ATSC HDTV Tuner on your television.
Another way to check is if the channels go into a -X or .X ending... for example, the analog channel 6 will be on Channel 6.. the Digital (or HD) Channel 6 will say 6-1 or 6.1 (or 6-2 / 6.2).
If your TV doesn't have digital channels you can always buy an Analog to Digital ATSC Converter box for under $60. I highly recommend the Lutema AirTV box. This digital converter box essentially acts as an HDTV Digital Tuner... giving you all the -X or .X digital channels in High Definition.
I've tried many of these digital converter boxes, however, the lutema one is by far the best one. It also allows for recording of TV shows right onto a USB thumb drive or an external USB Hard Drive. You can even record those superbowl commercials in HD right from that box.. for under $60-$80. Its Amazing....
Even if your TV has a digital tuner, you may want to consider the Lutema AirTV box and an HDTV antenna from BestBuy as it may pick up more channels than your older TV set due to enhanced frequency technology over the last 3-4 years.
I hope this helps and let us know how it goes.