what is the difference in performance between a fx 5200 128mb or 256mb
It'll depend on what games you are wanting to play, but the GPU speed is the same on both cards.
In most cases you'd be better off getting a faster speed GPU with 128mb of ram over a slower GPU card with 256mb.
The major difference is that the larger amount of memory on the card will be used mainly to load textures faster since they will be loaded into the graphic card's memory instead of the PC's RAM. But with a low-profile card like the FX5200, it doesn't make much of a difference, since the increase in memory will only give around a 5% increase in performance. Also, like DonNagual said, it'll depend on the games u want to play. It may help in Oblivion and FEAR but you'll get pretty much the same performance everywhere else. Graphics card memory alone doesn't dictate how good a card is. To illustrate an example of what DonNagual is talking about, the 128MB GeForce 6200 can blow away a 256MB GeForce FX5500, even though the latter has more memory.