Use the Crucial tool again as it will tell you what your maximum Ram capacity is + tell you how to achieve it, this should be by removing the 2 X 1GB sticks and replacing them with a further 2 X 2GB sticks, no need to purchase from Crucial simply write down the recommendation then shop around for the best deal, let us know the spec and will also have a look for the best prices.
Thanks for the further screenshot which does confirm that the system is presently using the MBs integrated video which is why the gaming is not as you would like.