Ram Me

It is beyond my meagre ability to understand why on earth the RAM in your video card should make any difference to your renders. But it does. Big time. I've been getting the "The effect 'Looks' failed to render..." and so I've had to unplug one of the monitors and render in 8-bit.
It's also beyond my understanding how my dual-quad-core Mac has a paltry 256MB of vram in it. Or... wait a minute... why does this computer only have 4GB of regular RAM? Crap. That means something happened. There should be at least 8. I thought I bought 12...
Poopity.
I bet those problems are related.
Here's an article on upgrading the video card on a Mac.

My dual-quad-core Mac is serial number NQ83601AXYK
It has an ATI Radeon HD 2600 with only 256(!) MB of VRAM.
CoconutID says my Mac was built in 2008. But the model identifier says it's a Mac Pro 3,1 which this site claims will not work with the ATI Radeon HD 5870
So the ATI Radeon HD 5770 is it.

The Apple versions of that card are, of course, upwards of twice as expensive as the PC versions. But we can't render out movies in 10-bit with Magic Bullet Looks without a better card. So that's $250 right there.
The other major issue with this dang computer is that I've always had trouble with the RAM. System Scanner tells you what sort of RAM you need.
In my case it's DDR2 PC2-6400 • CL=5 • Fully Buffered • ECC • DDR2-800 • 1.8V • 512Meg x 72
That's another $250 for 8GB of RAM.