Both are a big step up from a an fx5200.
The 6600Gt is a lot faster than the basic 6600......if you are a gamer then the 6600GT is the card you should go for.
The amount of onboard memory will not make a difference on those cards.128MB is more than enough.
Check the power rating of your psu......the 6600GT needs a decent quality supply of around 350W.If yours is less than 300W then you might be better looking at the basic 6600 model.
A bit more information on the spec of your pc and what you do with it would help us clarify things.