Just cuz u upgrade to a 256, or a 512mb card doesnt mean ur gettin an improvement in performance. What u want is to upgrade to a more powerful card (not how much ram it has). For example, an x700 64mb card is 100 times faster than a 9000 with 128mb, so ram has very little to do with performance.
With that said, I dont think the z71 gpus is upgradable, as 99% of laptops are not. So ur pretty much stuck with what u have.