If you are the happy and lucky owner of an Athlon64 (i.e. Opteron - transformers, robots in disguise!) - then you will be pleased to see here that nvidia have released drivers especially for you! Just appeared today, apparently.
Of course, from my understanding, you would have to be part of AMD's developer program to get hold of an Athlon64 and compatible motherboard.... Surely?? But then as Slashdot.org comments, maybe there must be more of these babies kicking around then we all reckoned!
More here and here.
Cheap 64-bit computing! Hoah! Yeah! Kick ass!