I can see MPEG compression artifacts on DVDs, too, so saying "DVD quality" is a little dubious. I have DirecTV and it looks about as good as my friend's digital cable, both of which look much better than analog cable or broadcast. Haven't seen DirecTV HD, but HD digital cable is fantastic. Locals don't look so good on DirecTV and there's no locals in HD for a couple more years, though, so if you watch a lot of broadcast, it's probably not the best option.
Generally, though, SD on bigger sets just shows you all the limitations of SD. I'm pretty sure you'll be longing for HD on your 55". Might want to keep Voom as an option, although DirecTV will be dramatically expanding their HD lineup in the coming years.