I installed the 32 bit version of Mingw 4.7.2 (using the installer) on my Windows 7 64 bit. I use MinGW in an Eclipse C++ project in order to build a .dll file. So far ever
I recently faced the same problem, installing the MinGW-64 version enabled the -m64 flag for me. You can get an automated build from here.
MinGW-64
-m64
EDIT : Some guy (rubenvb) made some good job in the Personal Builds :
rubenvb
Personal Builds
There's gcc 4.7.4 here and even 4.8.0 here.