Is it possible to upgrade ATI radeon HD 2400 XT from 128MB to 512MB?

Hi, I just bought "Tom Clancy's Splinter cell: Conviction" and it doesn't work on my iMac.

Of course I should have checked in depth the details of the requierments because now it appears that my graphic card doesn't support the game.

(I didn't think it would be a problem since I play "COD Warfare" with no lagging or any bug at all)

I have an iMac 20" - Mid 2007.

Hence my question stated in the title "Is it possible to upgrade ATI radeon HD 2400 XT from 128MB to 512MB?"...

I should add in case of a positive answer: "and how?..."

Thanks in advance!