1. Consider building a machine with a dual core or quad core processor. The multiple processing threads will be very useful when recording multiple programs and watching a program at the same time. A fast multi core processor is a must if you plan on watching and recording hi definition TV.
2. Don't skimp on the RAM. A good media center will have at least 2GB of ram and should probably have up to 4GB. The ram will help keep things moving quickly while you are recording programs.
3. You can never have enough hard drive space. With high definition TV becoming more of a standard you are going to want to consider having large amounts of hard drive space on your machine. I wouldn't build a machine that had less than a terabyte of hard drive space.
4. To increase performance considering using a raid configuration for your hard drives. Using raid zero along with high speed SATA hard drives can help you improve video performance and can prevent recording errors.
5. Get good TV tuner cards with built in hardware encoding. Tuner cards with MPEG hardware encoding will take a load off your processor when they record tv shows, this will help ensure that you won't get any skipping or problems in your recorded output. I use Hauppauge cards in my machine and they work great.
6. Invest in a good decoding program. The software decoder software offered by Nvidia does a good job on my media center. However, I tested four or five different ones before I found one that operated the way I wanted with my hardware configuration. Be prepared to test some different options before you settle on the one that will work best for you.
7. Don't forget to order the remote control. A media center without a remote control is a travesty. These computers are designed to operated while you sit back on your couch. The remotes don't cost that much and they work great.
8. Buy the best video card that you can afford. The better the video card that you have the less chances you have that you will have problems with playback. Before you buy your video card also make sure to check the list of Media Center approved cards. If the card isn't on there you might want to avoid it. Also make sure that you buy a video card that has a high definition output that matches your TV (either DVI or HDMI are the best).
9. Get a good audio card that is capable of feeding source audio to your receiver or your speakers. If you plan on outputting 5.1 or 7.1 data from DVD's you are going to want to make sure that the sound card that you pick has an optical or digital coax output. You might also consider buying a card that will convert all the sounds used on your computer into a 5.1 or 7.1 digital audio feed.
10. Be prepared to play with your configuration and build. Unfortunately, building a media center is not an exact science and there is usually quite a bit of tinkering involved in getting a stable fully functional system. If you aren't comfortable playing with the settings and the drivers to get things to work you might consider buying a prebuilt media center and have someone else do the testing and configuring for you.
I have been running windows XP media center edition for quite a while now and I love it. I have built two different media centers and they have both been a blast. Try it for yourself and see.