You don't got it.
I had experienced with issues on SDTV, like what steel said. For PGR 3, I had been played on SDTV and their fonts are small and hard to see but graphic looks okay and not great as HDTV. I tried to hook 360 on HDTV with component cable and looks MUCH BETTER than SDTV because more lines and resolution. No wonders, it was changed since more games aren't catch on SDTV but I can careless about SDTV and Steel had same problem with Dead Rising and other several games. For Xbox and PS2, they are support progressive scan with component cable but never played with that and some Xbox games are support HD as well.
I was stated that HD is more worth for me to play games and most games are support them, almost all on 360 and resolution looks much better than 480 since more games aren't looks so good on SDTV.