In my opinion, an HTPC does not game. It's strictly for media playback. However, many people here have decided that an HTPC can do the same as any other PC, but placed in a Home Theater environment; hence the gaming quality of some HTPC builds.
Since this is what you're looking for, the best advice I could give would be to build the best single GPU gaming system you want with the additional DVR capabilities (TV Tuner/Extra Storage Space).
Rule #1 for self-builds - Never purchase piecemeal. If at all possible, save up funds until you have enough money saved up to purchase ALL your components. This way you're not affected by sudden budget fluctuations (unexpected required purchases prevent you from your computer purchases) or price drops. The only exception would be if one of your less critical components suddenly went on sale; i.e. a computer case goes from $100 to $60.
For the gaming component of this build, I don't think anyone would argue with the Intel Core I5-3570, GeForce GTX670/680, 8GB of RAM, and a 80-120GB SSD as core components. Adding in the DVR capabilities, you'd just want to add a cablecard device like the Ceton Corp InfiniTV4 USB/PCI-E, Hauppauge WinTV-DCR-2650, or SiliconDust HDHomeRun Prime 3CC/6CC and a mass of additional storage space. These devices, when installed into your computer or computer network, act as cable set top box replacements. Which of these cablecard devices you get is going to depend on your DVR needs, but I'm going to lean towards the Ceton InfiniTV4 PCI-E card.
An HTPC working with two monitors, simultaneously, isn't something that I think will work well. Windows 7 only utilizes one audio output at a time. So while the wife could be watching a movie in the secondary monitor, the PC that the movie is running from will still respond to audio commands as Windows 7 dictates. If what you're working on suddenly requires the same audio channel that your wife is using to watch her movie, she'll suddenly start hearing your work's audio output, rather than her movie.
Now towards the end of the year, Ceton Corp is coming out with a device that should alleviate this issue. It's called the Ceton Echo. Essentially, this is an extender for any system running Window 7. However, when connected to a system with multiple TV Tuners, one tuner can be dedicated to the Echo. With the PC hooked up to one HDTV/monitor and the Echo connected to another HDTV, your set up will act as if you have two separate computers/tuners. Audio and Video will play on the HDTV for the Wife via the Ceton Echo, while you're able to keep working on your tasks via the HTPC.
Hope this helps.
-Wolf sends