ZFS is fairly stable on i386 now, so you should be ok regarding that. You should follow the guides for setting up ZFS as if it was an i386 machine with 4GB RAM I think, then your other programs can take advantage of PAE and the extra RAM. As I said, the link I posted mentions some other important points about PAE on FreeBSD, such as some drivers are not supported on PAE at all so take a careful look before you decide if its a good idea to proceed. I've not used PAE on FreeBSD so I'm not sure just how many common drivers are unsupported...
Andy.