Mac OS X virtualizes video RAM and will NOT partition it between two monitors, even if some older tools claim otherwise. Video RAM gets assigned to whatever needs it, so with two monitors connected an application can still use nearly all of your 256 MiB (minus the frame buffer for the second monitor, a few window images from Quartz Extreme, etc). Core Image and Quartz GL should be able to make good use of all the free VRAM.
MacOS 9 did split the VRAM between two active monitors. These different behaviors can be seen with "ATI Displays" on Radeon cards under both systems.