Sorry, you'll need to go with an ATI/AMD based video card. I *love* Nvidia based cards for gaming (I've upgraded through many and currently use a GTX460), but I've always used ATI-based cards for PVR's since "Theater Mode" mirroring is still available.
I'm currently looking for a replacement for the ATI HD3650 in my PVR as it's starting to crap out. Recently picked-up a GT430-based card for $40 thinking I could take advantage of CUDA as well as hoping it would do full-screen mirroring/overlay (I've also known that Nvidia stopped supporting full-screen mirroring a few years ago, but I hoped maybe that changed). But, alas, it doesn't and probably never will.
So, it's back to ATI...just wish the drivers were more stable (ATI/AMD does a horrible job at writing drivers). Will probably get a 5570 as it seems highly recommended for HTPC use. I have an older Nvidia 7900GS that I might try with older drivers...but the idle power consumption is just too high for HTPC use.