upgrading graphics?

Hi, anyone know if its possible to upgrade from having a shared gma chipset 64vram as supplied with the standard white macbook to a dedicated graphics card as found on the more expensive macbook pros eg, ati radeon x1600 or better?
Anybody out there asked apple if such upgrades are available or am I looking to buy a new machine?

You can not upgrade the graphics in a macbook

Sorry, as you already know, its shared, and you can't change that, but you can always improve the graphics by adding more RAM.

Not really, the integrated graphics in the MacBooks only use 64MB of RAM, so no matter how much RAM you have, it will only use 64MB of it.

That's not entirely true. The Macbook ensures a minimum of 64MB VRAM and caps it at that during startup, but everythign I have read says it can use more RAM as aplications demand it. In fact system monitor results bear this out - easy to get it reporting upwards of 128MB being consumed if you have 2G ram in your macbook.