AMD64 Kernel FreeBSD machines are good about autotuning. i386, on the other hand, needs to be adjusted manually.
1. Rebuild your kernel
a. Disable/wipe all drivers you are not using or at least those you are highly unlikely to ever use.
b. Add: options KVA_PAGES=512
c. Recompile/Install new kernel
2. Add these parameters to /boot/loader.conf
This will get the machine online without it crashing with vm.kmem errors. ZFS will bring an untuned i386 machine down to its knees with kernel panics quickly!
Adjust the above variables to your tastes. My test platform is a Dual Xeon with 4GB of RAM.