Just F-Zero GX was enough reason for me to upgrade to win7x64. If you are planning to be a heavy weight Dolphin user you can't miss the x64 version. Besides, most games (in Dolphin) benefit in some way or another from the x64 version over the x86 one.
Rig
*Corei5 3570K *Nvidia 9800GT 1GB DDR3
*Motherboard Asus P8H77M-PRO *Win7 x64
*RAM 8GB DDR3(1600)