Personally, i would not get the FX5200. It's the bottom-of-the-barrel when it comes to video cards. you should be able to get a better card for about the same amount of cash.
As for your question, i don't think there would be much (if any) noticable difference between the 128mb and the 256mb card.
I had an FX5200 and it was a poor performance card for gaming.
Where do you buy your parts from? I like Newegg but they don't ship to the UK. (assuming thats where you are because od the "pounds" thing)