Is it worth the upgrade or should I just leave it?
What you have you wont see any gains what so ever. A fast graphics card doesnt improve on what you have already. It it too old and outdated to be worth it. Being totally honest spending £200 on a brand new bargain basement computer would beat any expense spent on upgrading what you have. However if you came across a fx5200 on one of the gumtree type sites for a few quid it cant hurt to try.
Memory on a gfx card isnt what is important it is all of it and what it is attached to. It is like a fiat panda with a v12. Lots of grunt in one way but still a panda underneath and let down by the unchanged parts