Very high-end HDTVs have upconverters built in. Recent developments in DVD players, however, make it possible to upconvert DVDs without spending huge amounts of money. You still need an HDTV, but it doesn't have to be top-of-the-line. Premium DVD players that cost a few hundred dollars extra can do the upconverting themselves.
If you're not ready to upgrade your DVD player, you can purchase a consumer-grade, standalone upconverter unit for a few hundred dollars.
There are also standalone HD-upconverter units intended for film production and mastering professionals that are used to convert their footage into a high-definition format before it is mastered onto DVD. These converters are very expensive -- they can cost in the area of $10,000 or more. Some of them are marketed as super-high-end home theater gear, even though the price point is more compatible with commercial use.
For more information on HD upconverters, HDTV and related topics, check out the links on the next page.