Go here: BrokenThorn Entertainment
You can develop everything from inside win7x64 with visual studio
If I were you I'd start with x86, you'd have to start with that anyway since you change the processor mode from x16 to x86 and then ultimately to x64. You'll not start in x64 or x86, you need code to switch.
My devchain was:
NASM => Bootloader ( or use grub )
VC++ 2005 => Kernel
Manual c+p => Osimage ( BL + kernel )
Bochs => Emulate
Start over !!