I don't do nVidia although I do MediaPortal and WMC and use $20 HD4550 & HD6450 with great success. Neither of them does MPEG-2 acceleration (ATSC tuning, DVDs, etc) although they are really fantastic at .264 offloading as long as the codec is DxvA as would be the case with the Win 7 MS-DTV codec, and the one in MPC-HC as well as its stand-alone filter you can use with MediaPortal. I playback MPEG-2 using 25% of my BE-2350 dual-core and 10% of my AMD 3.48GHz quad-core in terms of CPU. For 1080p .264 it's in the 15% (BE-2350) and 2% (quad-core) CPU usage. Also I shy way, away from codec paks -- sorry Shark -- as they're GPU intensive and I'm happy with the basic codecs in virgin form!
It's my understanding the nVidia is equally equipped so having DxvA codecs in use would be key!
In every case having a bunch o' Post-Processing sent to the GPU will eventually max it out. As far as my AMD I set no edge enhancement, no auto-contrast, no... no.. no... and all is well -- I mean, if it's all digital why should I need all this post-processing??? It's sorta like doing a bunch of "sound fields" in your AVR when you give it a prestine Dolby-Digital audio stream!
When you add a video card, be sure to disable your on-board video and free up any "shared" RAM... this also can increase your WEI on the RAM as a side point!
BTW, you didn't say what you were concerned about WITH your video performance in the OP.
Any updates to report???