gromit,
I run Pelle's C on Windows 7 64-bit. It works just fine.
To answer your numbered questions:
1: I would think so, but I don't program for a pda so I can't say for sure.
2: Yes.
3: Generally no, but, for example, if you are using an int or a long for a pointer or a handle you would need to change that because pointers and handles must be 64-bit.