Buildworld times on a AMD64 processor

[SKU]

Greetings!

Can someone tell me how much quicker FreeBSD runs on the Amd64 platform for
compiling worlds and or make release?  Currently I can build release on a 
Pentium 4 3.02 ghz in about 40 minutes (without ports) and would like to 
get this down to around 10-15 minutes (or even 20).

Second question: If you had to buy a couple of FreeBSD builder boxes for 
building releases and testing only, which platform would you go with: Amd64 
or the Opteron?

Third question: Can the x86 codebase be cross-compiled while running
under 64 bits?  Ultimately I am making x86 releases.

Thanks in advance for any help that you can offer!

-SKU