While you can virtualize, pfSense, I would strongly recommend against it (assuming you are using pfSense as your firewall).
A firewall is expected to be a hardened machine, capable of withstanding probing attacks from the world at large. On the other hand, VMware (and other hypervisors) is not generally used in that kind of environment, so it's significantly less hardened to external attack. When you virtualize pfSense, you add an additional weakness to your firewall, in that you are exposing at least part of the hypervisor (its networking) to the internet. In theory, if you configure everything correctly, you'd probably be fine. But there's a not insignificant change that you'd either configure something incorrectly, or someone could find a bug in VMware's networking stack, and you'd suddenly be exposed to attack.
The other downside to virtualization is that it adds another layer of complexity to using FreeNAS. You'd probably want a minimum of 32GB of memory, even if you are only virtualizing FreeNAS and pfSense. You also need a storage controller to pass through. I believe, but am not certain, that you could do that with your LSI 2308, but it's something you'd want to confirm. If it doesn't work, you'd need to get a PCIe controller. You could probably repurpose your SSD to be your VMware/hypervisor boot disk and VM store, but I'd strongly recommend getting a second for a mirrored config, or else you're going to be really unhappy when it breaks.
If your concern is power consumption, there's a bunch of mini-pc boxes out of China that have an insanely small power footprint, and are more than capable of pushing 250+Mbps (depending on what exactly you're doing with your firewall).
Some examples:
And there's always eBay for some last-gen boxes, like D525, which can be had for both insanely cheap, and are insanely power efficient.
