You can plug three Ethernet cables into the C7, and three into the Actiontec - the three attached to the C7 will connect to one another at gigabit since the C7 ports are gigabit, while those on the Actiontec will connect at 100Mbps to one another or anything on the C7 since the Actiontec ports are only Fast Ethernet (100Mbps).
You can turn off the radio on the Actiontec or leave it on and have a radio for old B/G connections but you will primarily use the C7 radios as they are much better. You just have to insure that the 2.4GHz radio on the C7 is set to a different non-overlapping channel than the Actiontec, so choose from 1, 6, and 11 between them.
You won't notice any difference using either of them as a router, as home use is not very intensive.
If you wish to use bridge mode and see how well it works for you, follow
THIS guide.