Post about the various Intel (generally better compatibility) based cards
UK readers: this card appears to be very popular, see reviews
Many posts about fitting a HP NC360T Dual Port Intel based card in the PCIe X1 slot
For 10 Gbe, the Intel X520-T2 server adapter works. Initial iperf tests show the throughput at 3.5-4 Gbps, though this could probably be better through some tweaking since it doesn't seem to be pegging the CPU on either the server or client end. I'm unable to comment as to whether the low-profile bracket of the adapter fits since I only had a full-size bracket that I took off to fit the card into the N40L.
A cheap alternative for users wanting to boost the standard 1Gbps of the Microserver is the Intel EXPI9301CTBLK EXPI9301CTBLK.