6221 is indeed only 32 bit. 64 bit macines are probably the last major upgrade for awhile. The good news is that Sparks dev machine requirements should diminish going forward, making this a cheap hardware upgrade. Further information regarding the 2008 release is (only) available on the 2008 beta forum.
Can the CPUs be swapped out with 64 bit ones or is it a whole machine replace? Also, isnt the 6221 just 2-3 years old. Seems pretty quick for Autodesk to obsolete them. I have sent 2 email and 1 voicemail and still have not recieved the 2008 beta invite.