Tbh you sound like you would rather stick with Nvidia seeing as you have ignored the 6950
The gtx 480 would be quite a bit louder than your 9800gt though it does depend on the cooler on your current card but personally I would avoid unless you get an aftermarket cooler.
If you want silence then would recommend the 570 as they generally run quiet. Maybe a gtx 560 if you want to save some money.

If you want silence then would recommend the 570 as they generally run quiet. Maybe a gtx 560 if you want to save some money.