avil
The 1751 like other routers in the 1700 series offers a built in 10/100 Ethernet interface. And it offers a variety of cards to go in the other interface slots. Unfortunately it looks like the only Ethernet offered is the WIC-1ENET which is a 10 mbps card.
I looked in this link for the capability of the 1751 and you can check it again if you wish:
http://www.cisco.com/en/US/products/hw/routers/ps221/products_data_sheet09186a0080088719.html
If you really need 2 100 mbps interfaces then I think that you need to look at the 1800 series routers.
HTH
Rick
HTH
Rick