Yes you can replace your graphics card with the late 2009 iMac's Ati 4850 or newer Ati 5750 used on mid 2010 iMacs.
older Ati 4850 has 512MB of Ram and 256Bit Memory Bus GDDR3
newer Ati 5750 has 1GB of Ram and 128Bit Memory Bus GDDR5
It is your choice, but personally i don't prefer downgrade your memory bus speed to increase amount of ram. Bus speed is more important than ram size on older graphics cards.