Congrats on joining the 21 Century!
Once you get used to HD you'll wonder how you got along without it.
My set doesn't have Internet built in but I have a Roku box that'll stream video from Amazon, Netflix, Hulu etc and it's wonderful, perfect for watching the occasional missed show or a movie you don't feel like going out and renting.
I usually buy the SD versions of TV shows and they look fine, standard DVD quality almost though that may it may depend on the speed of your Internet Connection.
On FIOS it usually never takes longer than 4 or 5 seconds to load a show.