My brother's 360 is hooked up to a standard A/V input (obviously an SDTV). He hasn't gotten around to buying an S-Video cable yet.
With the exception of nigh-illegible text in Dead Rising, the graphics are still phenomenal. You can't really tell in PGR3 (honestly I was more impressed with Gran Turismo 3 than PGR3) but Chromehounds looks spectacular, as does Dead Rising. And the sheer amount of stuff that can be happening onscreen in Saint's Row is pretty stunning too, even if the graphics are a step down from other 360 titles.
and THATS the difference between HD and SD TV. PGR3 is head and shoulders above DR, GTR3 and Chromehounds. Chromehounds on HD actually shows many more flaws; SD hides some of it (not to take away from the game, but it isn't the most polished game).