I hesitated a bit to offer up Rodney's answer as I'm not sure that many consumer based ISP gateways have this level of granularity in the setup. Also a firmware change, hardware change, ISP change basically renders your network inoperable until you reconfigure that new ISP device (if it even has the same options).
Rodney is talking about basically trying to slice the pie, lets say your "cable router" has the lan site network configured like 192.168.1.0/255.255.0.0. The device itself has an address of 192.168.1.1. It has a DHCP server daemon that will hand out IPs in the range of 192.168.1.100-192.168.1.150. So that is your wired interface.
You can then setup your "wireless router" with an address of 192.168.2.1, just like a static host on your network. It will use the "cable router" IP as it's gateway. Then you can setup your wireless firewall to block traffic from 192.168.1.100-150. Depending on how the Network Processors are architect-ed, it could possibly bog down your wireless router and devices if it has to process a bunch of rules. Anyway it would hand out a small subnet of addresses maybe the private side is like 172.16.100.1/255.255.255.0 is the gateway and you are handing out 172.16.100.10-172.16.100.20.
It could all be moot and possibly your "cable router" will let you setup VLANs (virtual lans, with virtual gateways) May be called Zones or some other name that infers network segregation. All in all I would get a SMB sized Router from Ebay.