I say avoid a large collection. Even when HDwhatever is introduced, it will still takes years to get the entire back-catalog onto the movies. The first to come will likely be the action/adventure films, then comedy, drama, foriegn, etc etc...
So if you buy the LOTR: extended mega box, ultimate 10-disk Matrix box, the Rocky boxset, the Alien quadrilogy, and 6 Star Wars films, don't be disappointed when they all get the HD treatment by 1st Quarter 2006.
I've sold off my 300+ collection to under 100. I'd like to get 30 or fewer before I have to dump them at the FYE for $.50 store credit.
Exception: Alot of 4:3 TV programming won't look any better on HD (since alot is mastered on SD tape), so I'm not going to suggest you not collect TV series on DVD unless the show was native HD, in which case you'll wanna wait.