Apokalipse, why do you recommend voltmodding to people who don't know much about graphics cards? That's just plain risky...
In my opinion it's worth the extra money. Right now you don't need 512mb as 256mb will run fine, but firstly it's more future proof and secondly you can safely load the most high-res textures in your games. Besides, if you weren't a demanding gamer you wouldn't look at the 7950GT anyway.