You'd probably notice the biggest difference upping you RAM on your pc depending on how much you have already of course.. but 1gb should be enough.
VRam can sometimes be changed, but the rest of the board is hard coded to work with the original set-up and tailored to work best with that config, so an original 256 will always be better than a 128 thats been upgraded
256MB and the upcoming 512MB frame buffer GPUs are essentially just marketing hyperbole for the most part. Only the fastest video cards can really gain any improved performance with a 256MB frame buffer and performance increases are only usually noticeable when using high levels of advanced filtering such as antialiasing and anisotropic filtering.
An FX5700 is barely fast enough to utilise that extra memory effectively when put under the strain of using the advanced features that actually eat up the memory in the first place.