Between graphics card upgrades I expect the new card to be at least twice as fast as the old one.
So far my major graphics cards upgrades for my desktop have been:
1.)9500 pro
2.)6800LE unlocked to 16 pipelines and 6 vertex shaders
3.)x1900gt
4.)HD3870
5.)HD4830 - this was because I broke my HD3870!!
TBH even after selling the "old" MBP it will still cost a lot to upgrade to the newer model. It is pathetic that ANY expensive notebook like the previous generation MBP comes with 128mb of RAM for a dedicated graphics card. Even my first notebook from 2003 had a graphics card with 64mb of onboard RAM.