I recently put together a new Media Center PC.  The old one was running on a Pentium4 1.7 GHz and was becoming too slow for my tastes.  It ran okay but was becoming slow to bring up the guide, etc.  The day following Thanksgiving, I was able to score an AMD Athlon 64 x2 3800+ for a good price and decided it was time to put together the new machine.  Here is what I ended up with:

AMD Athlon 64 x2 3800+

1 GB RAM

NVidia GeForce 6600GT

NForce4 motherboard (ECS A939) - it was free with the processor.

Two analog tuners (one Emuzed and the other Conexant)

Silverstone LC14 case

This isn't the absolute fastest machine on the block but it has some getup and go.  I'd heard rumors that Media Center glitched on the dual-core processors but following the advice of David Fleishman, I installed new HAL drivers and haven't seen any issues.  The machine is definitely snappy.  I don't have it hooked up to an HD TV yet but it should be capable of doing WMV-HD 1080p decode with hardware acceleration through DXVA-WMV.

The case is quite cool.  It's a little tall but it has a nice black color and fits well with other media components.  Unlike some of its competition, it has space for more than 1 internal hard drive.  In fact, it has space for 3 of them.  It also has space for 2 CD/DVD drives.  The drive covers are better than most for HTPC cases.  They are flip-down doors, not stick-on cover plates.  The only downside is that there is no way to have a card reader in the case.  I suppose that is what the other machines on the network are for.

While I'm on the subject, I should mention that I'm using a Home Theater Master remote control.  This brings up an interesting issue.  Media Center doesn't work well with universal remotes out of the box because it has some technology to stop signal double-taps.  The behavior you'll get is that each button will work only once and then you have to hit another button before it will work again.  The solution is to follow the advice of Michael Swanson who outlines a registry setting that will diable this feature and let your universal remote work.