You could have a go at replacing the stock fan, or just buy a GTX 460 (as you probably should have done in the first place).
The Asus DirectCU, EVGA (with large fan), Gigabyte (dual fan), MSI Cyclone (768MB edition) are amongst the quietest GTX 460s. The quietest is the Zotac AMP Edition, if you can afford it or the extra cost can be justified.
I Got the 465 because I got a good deal on it. the 465 was cheaper plus it has more memory.
The Zotac looks great and I will consider it.