Home theater systems are not really suitable for karaoke, they are designed to play pre-recorded music, not the high transient signal levels of live vocals.
All depends on how much you are willing to spend and what your intended use is. Probably the best thing to do is buy a PA package like:
http://www.musiciansfriend.com/product/Phonic-Powerpod-620-Plus-S712-PA-Package?sku=630487
Some examples of systems are at:
http://www.musiciansfriend.com/navigation/live-sound-packaged-systems?N=100001%2b304603&Ns=P_Price%7c0&page=1
At that point, you can hook up your DVD or computer to it, and produce karaoke that can be done for a good-sized party or function.
If you are going to just practice at home and you don't care about sound quality, get a cheap karaoke machine like the VocoPro Duet or GigMan or something. Or even a magic microphone, which has given some people their start.