I use Windows 7 Media Center with a number of different tuner / capture devices. I have 3x SiliconDust HDHomeRun network tuners and 2x Hauppauge HD PVR units. SiliconDust - HDHomeRun Network-based Dual Digital - HDTV Tuner Ethernet InterfaceHauppauge HD PVR High Definition Personal Video Recorder 1212 USB 2.0 Interface
The HDHomRun can do either ATSC, Clear QAM or both (one per tuner). The HD PVR can record up to 1080i with 5.1 optical audio 100% DRM restriction free via component video (AVCHD / H.264). Being able to record HD from even premium channels in Media Center is no small thing.
Hauppauge has recently released the "Colossus" HD PVR which is cheaper and uses a PCIe x1 interface rather then USB like its predecessor:
»hauppauge.com/site/products/data ··· sus.html
As for motherboard and CPU, I started off using an AMD Athlon 64 x2 4800+ so most modern dual core systems should be fine.