Hi,
Did you ever get an answer to your question about the XFX GeForce 6600GT? I have the exact card and I've used it for 2 months now, no noise, no problems.
I, like you, wanted a silent box so I went with this video card, a Lian Li PC-V1200 case and a Zalmon CPNS 7000 Cu CPU cooler. It's very nice not hearing all of those fans noises and roar at night time or during the day.
BTW - I have also OC'd the card GPU to 525MHz from the stock 490MHz.
I hope this helps,