Onboard video cards don't "borrow ram" from the existing ram, it has its own. For example I know that the onboard video on my Mobo has 96mb of dedicated Vram. There is a big difference between the two.
It should be cheaper to get new RAM, even just double it to 1gb would see a pretty significant performance boost. A video card would make an impercetpible performance change for general web browsing type stuff.