If you've got an x8 slot to spare and don't mind running a copper SFP+ cable (limited to something like 5m, if I'm not mistaken), then you can get a 10G card stoopid cheap. Just saw one on Amazon for $32.
A lot of motherboards have x16 slots that only have 4 lanes connected. For a single-port PCIe 2.0 card, that seems like it should be enough by my math.
BTW, one thing to be aware of is how much power some of these switches use. Here's where SFP+ copper cables have an advantage over RJ-45 twisted pair. ServeTheHome has many small network switch reviews and they're pretty good about including some power consumption tests. I did already check, but failed to find a review of the switch mentioned in the article. It's likely they reviewed one or more other switches that have basically (or sometimes identically) the same guts.