VRAM is important for... apps that use a lot of VRAM. Most games don't need a lot, but a few do for textures (Doom 3 has some 'ultimate' or similar texture level that needs 512MB VRAM). Most pro apps (Photoshop, Final Cut Pro, etc) don't need a lot, but a few do (Aperture and Motion come to mind). Watching movies or viewing photos doesn't need much VRAM.
With a 30" and Aperture, I'd go with the 256MB option. If you use virtual desktops (finally appearing in Leopard), that will also increase your need for VRAM.