opterons arent made off of durons, they are basically the same now as the 64/FX line... also the AMD processor is a real 64-bit processor... it uses 64-bit long GPRs(general prupose registers) which are still useable as 8/16/32 bit GPR's...
im not sure about intels 64bit extensions but the true performance increase gained from the GPR's in AMD's new processors isnt from just becoming 64-bit long mode capable but from the extension from 8 GPRs to 16 GPRs, hypertransport, and on-die memory controllers... so just the addition of this capability should show little to no advancement in performance...
as for opterons vs xeons.... the xeons were gods of server/workstation apps til hypertransport came along, it allows for lightning fast communication between multiple processors, along with the on-chip memory controller opterons of the same price range have been beating intels in nearly every category including traditional intel hotspots...
also in reference to AMD/intel sharing technology i think its not so much an agreement between them as the technologies being created as standards instead of patents... such as hypertransport....
hypertransport started as a consortium back in 1998, all protocols and designs for it were made into standards by that consortium and are free information to anyone, so intel could take this technology with no reprisal... not sure if everything the chipmakers do is like this but it could explain alot.