I see from here that the latest version of VirtuaBox that can be hosted under Mac OS X 10.6.8 Snow Leopard is v4.3.40 here.
Regarding Linux Mint as a VirtualBox guest, do I want the 64-bit version of Linux Mint, or the 32-bit version? My mid-2009 17" MacBook Pro (5,2) is 64-bit (Core 2 Duo), but with a 32-bit EFI, I believe.