you will not need to reformat your hard drive, but changing graphics cards in my experience is something that needs to be done in a particular order, I do the following:
download the driver for the hd3850
download and install Driver Cleaner Pro
boot into safe mode
uninstall the 7600 via device manager
reboot back into safe mode
run Driver Cleaner Pro and clean out any nvidia files
shut down machine and put in your hd3850
boot into safe mode
install the driver for your hd3850
reboot and hopefuly your good to go
woud wait and see what others say though, I'm not a frequent graphics card swapper
[edit] when you use Driver Cleaner Pro, be careful not to select the nvidia chipset option incase you have an nforce motherboard (I forget exactly what it is called in the list in Driver Cleaner Pro, but I think you just need to clean the 'Nvidia' option and the 'WDM Nvidia' option - someone please confirm this for me please)