512MB vs 256MB

Might take a look at http://www.tomshardware.com ...they've got some good info on videa cards.

VAPCMD

...you'd have to say what you're looking to run or improve upon, and what particualr model you are considering for us to respond fully. As VAPCMD mentions http://www.tomshardware.com is a highly regarded site for benchmarking most hardware in a wide range of tasks.

More graphics RAM obviously means higher resolutions possible, with the limitation usually being the monitor you use. It's a mistake to use RAM quantity as a guide to how fast a graphics card is (especially in games), as higher resolutions, and more detail made possible by more RAM are what lower performance...not what raises them. It's the specification and model of the GPU (Graphics Processing Unit) that determines the speed and power, not the RAM quantity.

Here's the Tom's Hardware link showing the visual differences (or similarity, if you prefer) between 64, 128, 256, 512meg in Doom 3. This game and the hardware tested are all relatively old now (2004), but shows effectively what RAM does actually do.

http://tomshardware.co.uk/2004/08/09/matching_doom3_with_the_best_graphics_card/

GeeBee.