not that i have seen. i'm sure that games like doom 3 set to jesus christ can my computer handle this mode might use all of the 256mb on a Graphics card but other than that 128 should be more than enough. the performance to price difference just isn't there for me anyways. But Bottom line it all depends on what game you play and what tasks you perform. if you do video editing, then i assume you might need the 256, but i don't know. here is a basic break down of the 2 cards you mentioned you wanted to buy.
nVidia GeForce 6800 GT
nVidia currently has a slight edge in the graphics card performance war, thanks to their GeForce 6800 series, which represents the largest leap in game card performance to come along in years. They support DirectX 9.0c and Shader Model 3.0 and are available for both AGP and PCI Express. Note that the 6800 Ultra wants two auxiliary power connectors, so you might want to go with the 6800 GT, which only needs one. For those who can afford them, this is positively dreamy stuff.
nVidia GeForce 6600 GT
Although very similar to the 6800 cards, including DirectX 9.0c and Shader Model 3.0 support, nVidia has made some changes to make the 6600 series more affordable. They have fewer pixel pipelines, fewer pixel shader units, and the memory bus has been reduced to 128-bit from 256-bit, but they still offer exceptional game performance.