Even though I understand the TKaiser remarks for those who want to use these with a tight hi-speed connection between the cards, I'm not that worried about that for my purpose : each card would independent with light (but precise) synchronization.
I'm a bit more worried by the post about 64bit, implying that I'm ignorant about the 32/64 bit world difference. I know from a long time all of this and the difference between both world is not only limited to a memory size. In fact, for my application, memory is not the subject. I would use this kind of card for signal processing with intensive use 64 bit integer and double precision floating point instructions (SIMD or not). 64 bit core will make the difference.