How much memory can a 32 bit process access on a 64 bit operating system?

A 32-bit process is still limited to the same constraints in a 64-bit OS. The issue is that memory pointers are only 32-bits wide, so the program can't assign/resolve any memory address larger than 32 bits.

Nobody seems to touch upon the fact that if you have many different 32-bit applications, the wow64 subsystem can map them anywhere in memory above 4G, so on a 64-bit windows with sufficient memory, you can run many more 32-bit applications than on a native 32-bit system.

2 GB by default. If the application is large address space aware (linked with /LARGEADDRESSAWARE), it gets 4 GB (not 3 GB, see http://msdn.microsoft.com/en-us/library/aa366778.aspx)

They're still limited to 2 GB since many application depends on the top bit of pointers to be zero.

4 GB minus what is in use by the system if you link with /LARGEADDRESSAWARE.

Of course, you should be even more careful with pointer arithmetic if you set that flag.