Dual display and VRAM usage.

Question

Hi, back in the days I wanted a 12" iBook but couldn't afford one, last autumn thought I decided to get a Macbook Pro, the update came and...
.. it had 128MB vram.
I want to use dual screen and maybe even occasional gaming, I couldn't decide if this where sufficient for my needs or not so I waited for the june update.
The mid-07 MBP arrived and, it had 128MB vram AGAIN!
Why oh why Apple? Oh, don't answer that, it's because you're in bed with Intel and want to force feed us useless highest end CPUs. But all I care for is the VRAM, why do I have to pay a ****-of-a-lot for a 1/11th faster CPU?
Anyway, I tried to decide if I should "bit the sour Apple" and get the mid modell or just get the low one because that one is much more price worthy. A few days ago I ordered the low end one.
But then I've seen on barefeats that he says that a 30" ACD display would use 128MB vram of the 256MB vram modell, are vram always splitted between displays in OS X?
You see, if the extra 20" 1680x1050 display only use 1680x1050x(3 or 4) bytes that is ok, maybe even if it's double buffered (is it?), because then I will probably have like 100MB vram left for all my windows and for desktop usage that is probably ok. But if the vram is shared between displays and my 20" which I will use most only get 64MB vram that's quite low.
I know I can turn of the sleeping capabilities of the MBP and shut the lid and only use one screen but that's a quite lame solution.
I hate how Apple does this, and it seems to worry a lot of people. Some get the more expensive modell and hate that they had to do it, some doesn't, and some don't buy a machine at all .. I would probably have stayed in the last crowd.
256MB vram can't have cost them many tens of dollars, why not have that in all modells? Why is the middle one even needed? Just make 2.4GHz CPU and option in that case.
Anyway, does anyone know how much vram my screen will have available? Will 128MB work for me with all the regular desktop apps I will have running? Or should I really go with the 256MB one? The tricky part is that my MBP might be shipped today or similair.
I've called Apple support three times but noone of them knows how this works so they aren't for much help.
(Dell vostro 1500 sells here in Sweden for less than half the price of the mid MBP and comes way more balanced, you can even get a 1680x1050 screen. Sure it has 2.0GHz CPUs or whatever but I don't care about that. Where is the old "omg MHz myth"-Apple? Please understand CPU speed isn't everything. Instead of leading the MHz myth fight it ;D, start calling the CPUs T7300 and so on aswell ;D)

Answer 1

Hi Johan, actually the VRAM used by OS X is likely to be quite a bit more than just your screen resolution multiplied by 3 or 4, since each open window is composited offscreen for the shadow effects and for things like zooming (Ctrl-Mousewheel) and the "Genie effect".
I'm pretty sure the total VRAM isn't divided in half for each of the two displays though, as it will be allocated from the video driver on an as-needed basis. In practice, with both displays active and a few windows open, you should still have maybe 80-90% of your VRAM available for applications.
Games and other VRAM-heavy applications are a special case as these will typically use as much VRAM as you can give them, and swap out to system RAM via the PCI-X bus when they need more (and unlike swapping to disk, swapping into VRAM from system RAM is still pretty fast).