Heres the thing. Slower cards cant use the memory efficiently. Think of a 300mhz p2 system with a gig of ram. There isnt enough cycles to effectively process all of that data simotaniously to warrent the necessity for that much ram in a multitasking situation. It may help, but still your money would have been better spent on a 450mhz p2 and 512 ram.
Using the example above, is similar to saying 256vram on a 9600xt. Usually the 9600's core will buckle due to other effects in highly textured games before the rest of the ram can even be utalized in a productive, ballanced matter. Don't get me wrong, it will help, just your money is better spent getting a 128mb 9700np then a 256mb 9600xt.
The same can be said for anything below a x800p is today's games. Usually those cards have bigger bottlenecks then their 128mb of ram. In some situations, like doom 3 with a 9800xt vs 9800p, the extra ram does help. But, just in the last example, your money is better spent on a 128mb 6600GT then ann overpriced 9800xt. Seeing how the 6600GT is just a bit better then a 9800xt in most situations, the same rule generally applies. The cards seem to be ballanced with the amount of ram that they carry, and more ram would just make not quite as much of a good deal due to price as they were with only 128mb.
