You can use Visual Studio 2010 to assemble x86 and x86-64 code,also NASM assembler will suit your needs for 64-bit coding.You can check also MASM32 project i'am using it for simple assembly windows programming.
There are other assemblers too:
look here http://en.wikipedia.org/wiki/List_of_assemblers
I will also recommend you a good introductionary 64-bit assembly programming under linux
"Introduction to 64 Bit Intel Assembly Language Programming for Linux"