yes, it is possible.
set a budget, check your specs, find the suitable gfx card for your pc. put the discreet gfx card in the pcie x16 slot (usually the top blue slot) and connect your display to the gfx card's output. then download and install the updated drivers for your new card.
what are your full pc specs? what is your budget? what do you want to do with the discreet card?
for budgets $140~ - radeon hd 6850, $170~ - radeon 6870, $250~ radeon hd 7850 are good radeon cards.