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If you want a HTPC nvidia card, then I would strongly recommend the GT 430. According to this Anandtech review of discrete HTPC cards it is one of the very best. There are also passively cooled low-profile versions of this card - such as the "sparkle" one.
That said, before you put money down on a new card, may I ask what your current AMD card is and what specific issues are you having?
Whats wrong with AMD? What issues are you having?. I've ran a Radeon HD5450 and HD6310 in my HTPCs and both have been fine.
Would you be able to connect the PC directly to the TV and route the audio separately to the amp (ideally using a digital connection)?
What version of CCC are you using with the 6450? I found that 11.8 and 11.9 were best and most stable for my 5450. It seems just like an overscanning problem? I had a similar issue when I first set it all up but after setting underscan to around 5% sorted it. Presumably you've already tried going into CCC > My Digital Flat-Panels > Scaling options?
Interesting that Windows doesn't detect your TV but recognises your amplifier. My just states "SONY TV" with no mention of the AVR?