I found a link to this PDF on the Ubuntu forums, and I thought it was right up our alley here at Phoronix. It would be cool to have possibly a more detailed, verifiable benchmark performed along these same lines. Here' the link:
http://art-blog.no-ip.info/files/amd64vsi386.pdf

I'm going to install Ubuntu 64-bit once I have a weekend to mess with it. Post if this link stops working anytime soon... I'll put the file up somewhere and change the link.