Does NASM Do 64 Bit Assemblies?

I have nasm installed on my Fedora Core 4/AMD Athlon 64 X2 4400 box. I'd 
like to play with it, although I suppose I might be better off playing 
with C. Will nasm automatically default to 64 bit addressing? I was 
given Randall Hyde's "The Art of Assembly Language" book and finally 
noticed it is a 32 bit version.

Is there a better (open source) assembler for the Athlon 64 X2?

Thanks

Bob Cochran