You can normally get away with a straight swap and windows will ask for a driver or try to
find one for you.
If you wish to play it safe (highly recommended) :-
1/ Back up all your data files
2/ Go to Device Manager and change your existing
card to use the standard VGA driver.
3/ uninstall any software that relates to you
old graphics card
4/ Download a copy of the latest driver for your new card (with or
without catalyst) and save somewhere easy to find.
5/ Shut down your computer and
unplug the Mains Plug
6/ Swap the graphics cards (don't forget that some powerful
cards require an extra power connection)
7/ RePlug mains and start system
Install latest driver from step 4
Hope this helps