It doesn't realty matter, any motherboard that can handle games will usually have a on board NIC that will do just fine, some even have 2 nic.
If it doesn't then just buy one that is at least 100 Mbps, witch it is tough to find one that doesn't go the fast or faster. I would get one that supports Gigabit my self, there at newegg for like $30.
it should tell you, some where on the specifications,
something like "on board LAN"
and then tell you a speed rating
I always like pitcher of the board, and if it has a big phone jack looking thing the it has a lan
But like I said it is getting very rare to find a motherboard that doesn't have a on board NIC
Where are you buying your Motherboard? Or do your have it already?
this is my computer build that I will be buying off of Newegg(unless I find better price):
--Intel Core i5-750
--ASUS P7F7-E WS SuperComputer LGA 1156 Intel 3450 w/ NVIDIA NF200
SATA 6Gb/s USB 3.0 ATX Intel Motherboard
--G.SKILL PIS 4GB (2 x 2GB) 240-Pin DDR3 SDRAM DDR3 2000 (PC3 16000)
Desktop Memory Model F3-16000CL7D-4GBPIS
--Seagate Constellation ES 500GB
--MSI N260GTX Twin Frozr OC GeForce GTX 260 896MB 448-bit GDDR3
PCI Express 2.0 x16 HDCP Ready SLI Support Video Card
--Antec NeoPower 650 650W ATX12V / EPS12V SLI Ready
CrossFire Ready Modular Active PFC Power Supply
--Antec Six Hundred Black / Silver ATX Mid Tower Computer Case