I have to say, even a Prescott can do 100Mbit with ease. I used to have a rather old DDR2 system on a 100/100 symmetrical fiber somewhere which only got replaced after the (also ancient) mainboard got killed by the capacitor plague. That one also had a single ethernet port, and we used VLANs and a switch to bring both WAN and LAN and DMZ networks over that single card. But purely power consumption alone makes a direct-from-China box a better fit for most setups.
Edit: if you have some spare ports on your switch it might be worth just doing a setup on one of those (HP7800U) to try it out (just to play around with it).
- Create two VLANs on your switch, assign one port as trunk for both and two ports as access for either
- Put opnsense on a USB stick (or CD-R

) and boot from it (live boot works fine)
- Assign VLANs (you'll be prompted at startup)
- Assign firewall interfaces to those VLANs (WAN and LAN)
At that point you can plug a computer into the switch port you configured to access the LAN VLAN and try out the WebUI. Of course, mixing in VLANs and a single-interface network makes everything a bit harder to setup but at least you can do it with what you have on hand already.