Of course, to fully utilise the prowess of your HDTV, you need to watch something in HD, and that thing could be either a HD channel from free TV or cable, or from a Blu-Ray disc. I don't doubt the awesomeness of watching movies in glorious HD (e.g. the Blu-Ray's resolution is six times better than a DVD's!), but it's still too early to jump on the bandwagon. For one thing, the Blu-Ray library is currently very limited, so a lot of your favourite movies might not be available in this format for years to come. Furthermore, Blu-Ray disc prices are around the range of S$50, compared to Region 3 DVDs, often priced between S$20-S$30. A Blu-Ray player costs around S$600.
Unless you have deep pockets, it's better to adopt a wait-and-see approach. Prices would naturally drop once the Blu-Ray market widens in the next few months, and it's not like the DVD is immensely inferior to Blu-Ray. If you didn't know, there are Blu-Ray and HD-DVD players that can "upscale" your DVDs, or artificially process the video from your DVDs into near-HD video quality, so, don't sell away your precious DVD collection just yet. The best time to start is either in 2009 or 2010. By then, legal movie downloads might have already become the norm, who knows?
Personally, I think the jump to HD is largely unnecessary. Unless you want to examine every pock mark on your favourite celebrity's cheek, the video quality on DVDs are already sufficient. The only time your eyes need the relief of HD video is when you're gaming. Us gamers are all too familiar with the eye-watering blurry graphics of past-gen systems, but the recent crop of HD-supported consoles have significantly lessen the visual strain, and probably improved gamers' endurance for marathon gaming sessions (to the chagrin of some parents, no doubt.)
Note: I never mentioned about Blu-Ray's audio quality because I'm the kind of person who can't tell the difference between 5.1 and DTS except that one is louder than the other. Thus, the prospect of having 7.1 surround sound in my movies isn't exactly something earth-shattering to me. You might want to ask or check out an audiophile's website for more info on this.