I was trying to find a good distribution of MinGW-w64 under Windows (32bit and 64bit). I think TDM64 seems to be quite good now.
Website and download: http://tdm-gcc.tdragon.net/
1) It runs under 32bit (x86) and 64bit Windows (x64)
2) It is multilib enabled, which means that it can produce both 32bit and 64bit binary. The default is 64bit.
3) It is a native 64bit compiler, at least under Windows x64.
4) Easy to be installed -- it has an installer.