For now, the only company with "64-bit OS only" boards will be AMD with the afore mentioned Dual-Cypress "5990" parts that feature 4GB of GDDR5 memory.
huh i dont think thats correct, the 5990 is basically a 5970 that means the memory config is 2*2GB and wold be seen by 32bit OS just fine?

I could be wrong tough!!