Did you try setting the Tivo Series3 to native as I suggested above?
If FiOS is available in your area, it offers somewhat better SD picture quality than most cable providers. That said, the 1954 NTSC standard was never really meant for display on 50+" screens. NTSC was designed back when 20" (and less) TVs were the norm. That's part of the reason we have high-definition -- not simply to make the picture look incredible, but to more appropriately suit larger displays which are becoming more and more common.
SD zoomed or "blown up" on a 52" will never look as it did on a 32" TV, regardless of what you buy. But what SD are you watching? The local networks broadcast their primetime lineups -- which I am sure you record -- in high-definition. Cable channels like CSN/FSN, ESPN, ESPN2, TNT, FOOD, MTV, NGC, Discovery, HBO, Cinemax, Starz, Showtime, and TMC are offered in HD, with others like SciFi and History to follow later this year. Even the news networks are slowly making the move to HD -- NBC News launches in high-def next month.