You would want to connect to the DG814 alone, enter the web based configuration, and turn off its built-in DHCP server (the exact verbiage may vary, but there should be an option for that). You also should make sure that the IP address assigned to the DG814 is different from your other router and any computers. After that, it should be safe to use it as a plain switch.
My old Micosoft MN100 router had a specific option to function as a plain switch -- apparently MS anticipated somebody might want to use it for just that some day. I haven't seen that on other routers, but if you happen find such an option on DG814, then you should be able to select it and not have to worry about DHCP, IP addresses and other things.
Unless the ports on the DG814 are autoswitching MDI/MDI-X, you will need a
crossover cable to connect it to your other router.
It won't degrade your network performance.
