Eric,
That looks like the SIR-T451 model. An 8VSB (off-air) only model. Good for off-air only. It will not receive in-the-clear cable QAM signals. It has been around for two years or more. As such, do not expect too much from it's internal processing on digital. Two years is a long time ago in digital growth.
Your HP monitor is showing what looks like 16x9 HD to me via the RGB connection which the manual says it will do. So far, so good. Looking at the manual for this box, it says that it will not letter-box 1080i signals on a 4x3 so you are probably looking at the 720p signal.
The manual also says it will downconvert the HD to 480p via the usual video out connector for viewing on a conventional set in conventional definition. It looks like you can set the box to letterbox or "zoom" (a 4x3 center cut of the 16x9 to fill the 4x3 screen) via the remote on the 480p.
The RF signal will only be a 480p signal for a regular set. The manual is not clear if you can letterbox or zoom on this signal. I doubt it. It is also not clear if the RF signal will pass any of the digital channels as down-converted.
You can download the manual here;
http://www.samsung.com/us/support/do...ype=UM&vType=L
I picked up a full digital LG DVD recorder box a few weeks ago for $250 or so. It has the full 8VSB and QAM tuner for use on my basic cable. Composite, component and HDMI outputs. It's tuner is better than the LG 23" HD set I am feeding with the same full tuner. I am amazed how much digital Comcast is passing on this signal. Local 4x3 analog channels, local 4x3 digital channels, local HD channels, digital 4x3 versions of some cable channels (but not HD on those) and a bunch of music channels. I live in a valley too far from the xmtrs for off-air. DVD recording is in 480p no matter what signal comes in.
Welcome to the confusing world of digital,
Dave A