Some detail from Apple about External Monitors resolutions.
When the flat-panel display and an external video monitor are operating at the same time, each is allocated 32 MB of video memory in 64 MB VRAM systems or 64 MB of video memory in 128 MB VRAM systems.
Seems the desktop spanning is built in by default for the Powerbooks. The question is, does having only 32Mb allocated to each screen make a difference?
Update: According to this page:
WHAT ABOUT QUARTZ EXTREME? Some of my programmer friends theorized that the extra video memory should speed up Quartz Extreme functions. I disagreed. I argued that graphics processor speed is the key, not graphics memory size, since Apple’s documentation states you only need 16MB of video memory for Quartz Extreme functions.
I saw an interesting graph on Apple’s Quartz Extreme Page from a benchmark called “Window Move.” I was able to get a copy of a similar application that creates hundreds of buffered, semi-transparent windows in various sizes, shapes, and locations.
This test showed no difference.