I am running the 2-slot chassis with HD2, on the new 8-core Mac Pro 2.26. No problems, everything seems good. I have HD3, but no longer see the needs to go beyond 2 cards, as the Mac Pro totally slays the third Accel card. All three of them, actually! Selling my third card will pay for the chassis.
The Magma people told me that the 2-slot chassis is 64-bit, whereas the 4-slot is 32-bit. This supposedly would manifest itself as "slower", possibly meaning more latency. I don't have both to compare, so I have no comment. I will say that there is maybe the tiniest bit of latency with the 2-slot, or at least I thought there was. I can't hear it any more if it's there. Yesterday I did a session with a singer who I have been working with for the past ten years, and he didn't hear it