Assuming you wire the visitor AP to the rest of your network and have it on a different wireless channel, you shouldn't see any noticable degradation in your own network.

My setup is a dual-band router with 802.11n at 5 GHz and 802.11g on 2.4 GHz channel 6, with my visitor AP serving up 802.11g on 2.4 GHz channel 1 (too much interference on channel 11.) I've noticed no problems after adding the visitor AP, but then again, almost all of my devices are hard-wired or on the 802.11n network at this point.