Well, when do you really need 256MB of graphics RAM? Most gamers' computers only have 512MB of regular memory. So you're basing the price of RAM alone, and not some of the other neat features that ATI's have. I can't give you specifics of comparisons, because I don't know them.
I DO know that my ATI supports Triple Buffering, 6x anti-aliasing, AGP 8x, DirectX 9 (native), has a faster clock speed, and outperforms most GeForces, even though it only has 128mb of Video RAM. Besides, if you run out of video RAM, you have an AGP Aperture for spillover, so it'll use your main memory...not as fast, but you'll rarely (very rarely) need more than 128MB.