The problem with that approach (and the main reason I haven't seriously considered it) is that watching those few high def DVDs might spoil you so that all your standard DVDs look bad. I already find it hard to watch a poorly mastered disc after seeing several good ones.
That is so 100% true!
Now a lot of the newer SD DVDs, especially dual-layer, look pretty good upscale to 1080p. But yeah, I tried to watch the HBO's Sopranos on SD DVD one time, and it looked so TERRIBLE, I had to just turn it off!
But the newer SD DVDs like Rome, Tudors, Deadwood, The Unit, Heroes, Lost, etc., look pretty good.
That is the price we pay.
I just saw "Brotherhood of the Wolf" HD DVD (UK). Compared to the SD DVD version, it looked a little better. But compared Pan's Labyrinth, DH 4, Bourne Ultimatum, etc., some of these so-called HD movies don't even look that great. So when we compare a great Video/Audio quality HD movie to a mediocre SD DVD, it becomes unbearable.