the amount of memory on a graphics card has little to do with its performance. what you look for, is the graphics processor speed, and most importantly, the number of pipelines the card has for transporting the graphics pixels to memory.
for example, if you have a graphics card with 256mb of memory, but only 2 pipelines, links redraws will not be very impressive. but you might have a graphics card with 128mb of memory, and 8 pipelines, and the card will give you some serious performance.
best thing to do is goto the manufacturer's website, and compare each cards' technical specs side by side.
edit: my favorite place to read about graphics cards:
http://forums.guru3d.com/