Aside from speakers, a decoder and amplifier are needed. While these two items are often found in the same box, called a "receiver", they are actually two distinct components. The amplifier is the more important of the two, and care should also be taken when choosing one of these. Again it is recommended to listen to different amplifiers before making a final choice. Depending on the hardware you may already have in your home stereo system, you may not need to buy too much in this department. If your amp is already capable of supporting the six channels, you need only buy a DD5.1 (or DTS or both) decoder. Most of the time, it is easy (for little added cost) to find a player that can output six analog signals, and need only pass through an amplifier. Cheaper DVD players do not have a decoder built in and merely pass an analog signal out to an offboard decoder for processing.
All of this, of course, adds more expense. Most reasonable midrange A/V receivers cost in the $500 price range, but as any audiophile will tell you, you get what you pay for. High-end amplifiers certainly cost quite a bit more, and could run in the $1,500 price range. Now between the speakers and the receiver, you are looking at a minimum $1,000 investment, just to get the digital sound with your movies. Now of course, if you already have some of these components, you may not need to spend as much. The other option is forgoing the sound. Now if you ask any DVD connoisseur, you will find that forgoing the digital sound is almost as bad as watching them on a PC monitor.
Of course there are other options. Some PC speaker setups now support Dolby Digital decoding for a reasonable price (under $300), and will certainly be a step up from stereo playback. To take advantage of this, however, a card that passes out an AC-3 stream will be needed (in other words not your standard video card.)
The video quality on DVD discs is also superb compared to what most of us are used to. While VHS tapes degrade in quality with successive viewings, DVDs never lose their original quality. DVDs are also higher quality than regular TV broadcast because of the fact that analog signals are extremely susceptible to noise. DVDs for the most part only use about 500 lines of resolution, and therefore are slightly worse off than digital TV (HDTV), but still use the same encoding algorithms - MPEG2.
When it comes to choosing a player, most are capable of displaying about 480 lines of resolution and, for the most part, all produce a similar picture. Things to be wary of when you are buying a set top are lip synch problems and artifacting (when there is a lot of movement, things seem to leave trails). Make sure you scour newsgroups and also see the player in action before you shell out the dough for them.