Nope, you'll need more RAM as well. With more RAM and a new GPU, you might be able to play some of the less intensive games, but it might be that your power supply can't handle a graphics card. That said, what is your power supply?
In that case, a new PC would be a good idea; It only supports DDR2 memory, making buying new RAM pointless since it can't be reused if you ever upgrade the motherboard,