I'm going to guess you have the Bestbuy version since you stated your Asus g51vx-rx05 will not accept a quad core CPU.
The RAM cannot be upgraded to more than 4GB, that seems to be the limit for this laptop.
Not sure if the video card can be upgraded. You will need to contact Asus about that. However, that's the easy part. There more difficult part is finding a laptop video card since they are not sold to the general public. They are only sold on to system builders, but perhaps you can find a faster card on eBay.