As far as I'm aware, the Pro version only has 4 MB (that's MegaBytes) of RAM, enough to play the smallest commercial games, which range from 4 MB to 32 MB in size. Most homebrew is smaller than that though, which means you'd be able to run it.
I don't see why they would add RAM to the M3 Pro, as it would be very pointless since they're already selling the M3 Perfect.
Basically, if you want full GBA support, and RAM for usage with DS Homebrew, you need a Perfect. The Pro can give you some GBA capability, but 4 MB really isn't that much.
Perhaps you're confused about the difference between bits and bytes. The Pro has 32 megaBITS like you said, which translates to 4 megaBYTES. Official GBA games can be up to 256 megabits = 32 megabytes.
EDIT: ah, I see the source of confusion. Looking at Gameyeeeah, they seem to imply that you can use GBA Tool to somehow make larger GBA binaries run. Unfortunately no, I'm pretty sure the option they show in that screenshot is only for fixing an overdump, meaning that if the game is 32 Mbits to begin with and you've overdumped it so that it's 256 Mbits with mostly empty data, it can trim that down to the 32 Mbits the file originally is supposed to be. Files larger than 32 Mbits to begin with won't work on the Pro at all, even using this program.