Comment by zamadatix

PCIe is also a full duplex connection so 2x10G is still just 20G instead of 40G. For PCIe 2.0 an x8 connection should get you full bandwidth on both ports simultaneously while x4 will fall just short for simultaneously usage (but still higher than 1 port). Unless you're really hankering for that full 20G, in which case a 25G NIC is definitely the better pick, that means you can just slot it in an x4 slot off the chipset on even a standard desktop PC.

Funnily enough, if you want a dirt cheap PCIe 3.0 based card the MCX353A-QCBT and MCX354A-QCBT give 1/2 ports of 40G QSFP+. They support QSFP+ to SFP+ adapters, so you can plug a 10G SFP+ into the QSFP+ port, but they don't support 4x10G breakout unfortunately. I ended up using the 2 port variant in both of my NASes - one port is 40G between the 2 for dirt cheap fast backups and the other is adapted to 10G to connect to the rest of the home network.