Right now I think the Nvidia GeForce 5950 256 MB is one of highest they sell (for high end prices as well..) But 6800 coming out soon, so that will be a lot better competition for the ATI. I'm not really a pro on the subject tho, so dont listen to me.
P.S. Not very much diff between 128 MB and 256, other than big price difference and a bit of FPS in games somewhat. You mainly paying for bragging rights. A topic being discussed constantly is waiting for PCI Express to come out, as it is supposed to be way better than AGP. I personally prefer Nvidia cards over ATI, but it is purely personal preference. Dont want this thread to start as another vid card arguement..