The PowerEdge R210 comes with 2 network slots out of the box (+ another one for iDRAC).
Is there a way to add another card to have an extra gegabit ethernet slot?
I've put a few of those little guys in-- they're very cute little boxes and reasonably inexpensive.
Dell will sell you addt'l NICs at time of purchase. Otherwise, you'd need an Ethernet NIC compatible with the PCIe 16x slot. The list of NICs that Dell will sell at time of purchase is a decent list to pick from:
Any of those would do what you want and will physically fit in the box.