Windows Media Center is a decent HTPC/DVR suite. If you're using Windows Vista or 7, it's included. If you're going to use some 'nix variant, you'll want to check on driver availability before you select any hardware.
For your digital tuner recordings, there's not much processing load, so Zacate may have enough grunt to do what you need. Otherwise, you could consider Llano. I'm currently using a Phenom II with integrated Radeon HD4200 (785G chipset) graphics for my HTPC/DVR. I intend to replace it with a Llano-based box next month. If you have old PC hardware available, you might use it for the guts of your DVR. This could be an excuse to upgrade your older gaming PC.
McTavish wrote:( I would have to run an ethernet cable from my bedroom PC to the entertainment system)
An in-home WiFi connection from your router or access point will be plenty fast enough for the periodic guide updates that your DVR requires.
i7-4770K, H70, Gryphon Z87, 16 GiB, R9-290, SSD, 2 HD, Blu-ray, SB ZX, TJ08-E, SS-660XP², 3007WFP+2001FP, RK-9000BR, MX518