If I send two signals, one to each router, will the 10/100/1000 router suffer at all times? Or only when those wireless devices are connecting to the 10/100 router?
More aboutsplitting signals
Spliting the signal will cut into half. Lets say you have 100 Mbps (fiber optic) coming from the Modem each router will get 50 Mbps. A much better and clean set up would be to connect the the wireless router to the Modem (router will provide same speeds to all devices) from there you will have four available ports that you can use for all wired devices and wireless as well. In the case that you need more ports just plug in a switch to one of the four available ports on the router.
Note that wireless speeds will depend on waht kid or router your use if you have G router you'll get up to 54 Mbps but if you have an N router you can get up to 150 Mbps.
So you're thinking:
And in regards to your note, N routers can access much greater speeds that 150 mbps.