My advice is that if you definitely have NO plans to watch them in the future, sell them. However, I would not sell them just because HD-DVD and Blu-Ray are coming out.
That would be like selling your entire CD collection because SACD and DVD-A came out. (You know how that ended up. They are merely slightly better digital sources, and are, at the moment, pretty much niche formats just for us Audiophiles. And who knows if either, or both of them, will be around in a few years.)
However, my advice is that if you ARE planning to watch these again, I would hold onto them, and wait to see which, if either, of these two hi-rez formats wins the battle, and dominates the market.
It could be that Blu_ray wins, because it is the better format. Or, it could be that HD-DVD wins, because it is cheaper and is coming out first, albeit only by a few months. (Again, remember the Betamax vs. VHS video format wars? Betamax lost even though it was better and a smaller format.) Or it is possible, that one or both become niche formats, because people don't want to bother replacing their entire DVD collection, just to get a slightly better picture. Remember that the picture quality will definitely be better than regular DVDs, so it will be a leap forward. However, it will not be nearly so big a leap forward as the leap from VHS to DVD.)
I plan on waiting a good long time for the dust to settle before choosing. (Who knows, maybe a third format will join the battle and blow both of those two out of the water?)
My two cents worth anyway.
