<< I wonder how graphics resolution translates to TV scan lines. >>
Well, graphics resolution is a digital term and TV scan lines are analog, so it's an apples to oranges comparison. But roughly speaking, horizontal scan lines are roughly analagous to VERTICAL resolution in graphics terms. i.e., 640x480, where 480 would be similar to TV scan lines. Standard NTSC video can have up to ~550 horizontal scan lines. But as broadcast over the airwaves it's at best 320 lines. Standard VHS can do about 400 lines, and DVD can be up to 540. The 640 part of the 640x480 example is meaningless in TV terms, other than how it would relate by virtue of image aspect ratio or size. And of course, every TV in the US uses a refresh rate of 60Hz, well really it's 30Hz because only half the field (every other line) is drawn on each pass (called "interlacing".
In my opinion (I know others disagree, but they are wrong ), at this point HDTV is only for those who are rich and technically savvy, or even richer so they can afford to pay someone who is technically savvy or are rich enough that they don't care they are spending a small fortune on a device where all the applicable standards haven't even been established yet.