1) Any graphics card for a gaming PC will serve in an HTPC, after acknowledging power & noise concerns (as you have).
2) Therefore, consider what sort of gaming performance you will require from this machine, and evaluate your card choice as a gamer first. Things I'd consider in particular:
---a) Do I want to impress friends/family with how awesome my games look? (I'm serious. For real, I have roommates who are perfectly happy to watch me play Deus Ex, Mass Effect, etc. instead of watching TV.)
---b) What titles do I play now? What will I be playing in the next ~12 months?
---c) Will I be able to upgrade this card in the next 12, 18, or 24 months?
---d) How much time will I spend gaming on this machine, and how many dollars per hour am I willing to spend on increased performance?
For my money, I'd go for the GTX 670, but I'm picky about frame rates and my lifestyle is such that this system would do more duty as a gaming rig in the living room than an HTPC playing media. YMMV.