That would not be cost effective or even possible on this 7 yr old computer. Put the money towards a new computer. You do not have a separate graphics card. Everything is integrated on the board.
It's this board
However, as the other reply mentions, it may not be cost effective to replace the board on a system this old. You can easily find working, complete used systems -- many a couple of generations newer than this one -- for equivalent money on EBay, Craig's List, etc.