Oh sure, I could get a switch. But this is a home-based small business, so every penny matters, and like I said I’ve already got the NICs on hand. I also only need two client ports (one SFP and one RJ45) and one RJ45 uplink port. I can get a SFF PC with two PCIe x16 slots for as little as $65; the cheapest possible switch is the D-link at $200, and it only has two 10G RJ45 ports.
The only question is whether the i3-3240 in that $65 SFF PC is enough to forward traffic at anything close to line speed. I went ahead and bought it, because the seller has a decent return policy, so I guess I’ll find out!