What are the rest of her system specs? The only components producing noise should be the case fans, graphics card (unless it's passive, like her 6600 GT), power supply, CPU heatsink / fan, and hard drives. Most modern components are whisper quiet, unless otherwise stated around the 'net.
Is her case running a bunch of 80mm fans or a high RPM CPU fan? Also, is the noise a "air rushing" sound, or a clicking sound from a hard drive? Is the noise constant or does it increase / decrease with time and depending on what she's doing (such as playing a game)?
On a final note, to save you the aggravation, make SURE to do the following when you install the new card:
- Download an install a program called Driver Sweeper from Guru3D
- Uninstall Nvidia drivers from Control Panel
- Restart into Safe Mode (this is important)
- Run Driver Sweeper and have it detect and remove any remaining Nvidia driver components
- Shut down the computer, remove the 6600 GT, and install the HD 5670.
- Start it up and install the latest ATI drivers.