Before anything, I'd like to point out that I've read the SLI / CrossFire FAQs by Maziar (which was very useful btw) but couldn't find the info there.
I have 2 ATI Radeon HD 4670 in CrossFire, one with 512MB GDD3 memory and the other with 1GB DDR3.
My first question is if some game require a 1GB memory graphic card, would it be better to disable my crossfire configuration with both at 512 MB such that I have only the card with the 1GB memory or does the fact that 2 graphic cards working together compensate and even surpasses that 1GB DDR3 card alone in that specific case?
2nd question: How could that be done without going the hard way, ie, opening the casing and removing the crossfire link?
I've never tried that before, but you can always disable CFX in the Catalyst Control Panel by just unchecking that check box. With the crossfire bridge on, it will allow the box to be checked, otherwise it's greyed out.
Crossfire/SLI most common operation method is rendering one frame on the first GPU and the next one on the second. Thus, unless there is some sort of smart caching on the gpu memory, you will notice a slowdown whenever more than 512mb are required. If that is really the case, a 1gb card will probably work faster than 2 cards accessing your HDD or system memory.
Then again, as sabot00 mentioned, a scene requiring a 1gb frame buffer will probably stomp a 4670 anyway, so you're better of in CF.
Combining sabot00 and Murissokah 's answers, I conclude that it would be best to leave it always on CrossFire.
For those interested in the question, 1GB isn't all that useful for an HD 4670 since the GPU itself might not be able to fully utilise it. But I also read that when in CF, that 1 GB might now be better exploited.