Setting x86_64 architecture in Xcode

It seem that the default architectures for a new project in Xcode 3.0 with Leopard is i386 (at least for my Mac mini Core 2 Duo).
I found a way to set the architectures to x86_64:

Select project name in "Groups & Files" and click "Info" icon
Select "Buuild" tab then double-click "Architectures" line.
In the panel select "64-bits" and unselect "32-bits".
Double-click "Valid Architectures", delete all except "x86_64"
Switch "Configuration" (Debug or Release) and do the same change.
Select "General" tab and click "Rebuild Code Sense Index" button
Return to the Project window and for each configuration (Debug or Release)
set x86_64 in "Set Active Architecture" of the Project menu.

Compile and Debug work fine except Debugger window don't show any Registers
(but command like "info reg r10" or "info all-reg" in Console window can be used...)

Please if you have any hint to fix the Registers problem let me know how...