Not sure if this applies or not, but as Snow Leopard 10.6 will be hitting on the GPU for a lot of processing that is done on the main processor in 10.5, will the VRAM come into play in that process? If the answer is yes, then you probably would want all the VRAM you could get. Otherwise, with a very few application-specific exceptions, you won't find a bit of difference running most apps (including Photoshop) under 10.5 today. 3-D rendering applications (and probably games) are the main exception.