edietz, keep in mind that most hd televisions will include an atsc tuner as well as the ntsc tuner.
This atsc television tuner will allow you to receive digital signals over the air with the use of an antenna and some RG6 (or RG59) cable. This is the same type of black cable that you are using for your comcast cable tv signal now. The ntsc tuner allows our tv to tune our cable tv signals as well as the older analog over the air signals.
The great thing about the Over-the-Air digital signal is that it will probably be better PQ than analog cable, it has minimal cost (any decent antenna will do), your reception of these digital signals will include free HDTV from the major networks. Not all digital is HD, but some is.
Here in the greater Toronto area we are receiving about a dozen or more digital signals over the air. Some are local and national stations, several are U.S. stations: ABC, NBC, CBS, PBS, from across Lake Ontario.
In terms of programming you can receive all of the prime time network shows ( Law & Order, C.S.I., Desperate Housewives etc.) as well as some late night ( Leno, Letterman, Saturday Night Live). All in HD.
Completely legal and almost free. This is the same over the air stuff we had before cable television. You probably know this, but not everyone does and so I mention it.
In response to your question, the PQ will vary from bad to o.k. with cable television on a larger tv whether DLP, LCD, plasma, etc. Poor tv signals look poorer the larger they get.
I am going to respectfully disagree with supervij's recommendation. His points are completely valid and accurate.
However, if you and your "wife to be" are buying now, I suggest you spend on a HDTV. You will be receiving more HD content in the next few years and it would be a shame not to have the hdtv capability. Look at it as an engagement or wedding gift for one another. Alternately, wait a couple or three months while you research your purchase and watch the prices fall as time passes.
Good luck and good hunting!