Basics
     A set-top box is a "box" that sits on top of your television.  It is connected via cable or some other means and allows you some level of interaction with your tv.
 
 
Hardware:  Simply up the faster the better!  As the box we are building is driven by software the faster processor you have the  happier you'll be with the performance.  I've ran MythTV on a Duron 800 with 384 megabytes of RAM.  While this isn't powerful enough to watch live tv (remember the box is encoding and decoding at the same time), it does a great job of capturing mpeg4 at 480 x 480.  CPU utilization is about 70% and the majority of the RAM is in use.  Playback was nice and smooth!  I'm currently running MythTV on a Athlon XP 1800+!  I have two capture cards and am able to capture two shows at the same time!
 
My personal guidelines would be:
If you only wanted to record one show and play it back at another time a Duron 800 is fine.  I'd also recommend no less than 256 megabytes of RAM.
 
If you want to watch one show while recording another, you'd want nothing less that a 1.4 GHz CPU and 384 megabytes of RAM.
 
To record two shows at once, I'd recommend nothing less than a 1.8 GHz CPU with 512 megabytes of RAM.

My minimum recommendation for HDTV would be a Athlon XP 2800+.  This in conjunction with a video card that used the nVidia Geforce 5200 chipset and has 128 megabytes of RAM.  You'll want to ensure you are using the binary nVidia drivers and XvMC.
 
TV Tuner:  Hauppauge WinTV model 401 w/ dbx.  If you only have one card, it simple works right out the box(less btaudio).  You don't have to worry about passing tuner option to the kernel.  If anyone can recommend a card that work right out the box, please post it in the forum.
 
Soundcard:  Soundblaster Live!  Works great and can you hook it up to digital receiver!
 
Hard drive:  No real personal perference, but nothing less than 80 gigabytes.
 
Software:  Currently MythTV is the most mature project for a home grown PVR.  Check the  links for more options.  I've simplified the installation process with  KnoppMyth.