Actually, doing the three-router 'tree', you'd still want to have DHCP enabled on it (for its LAN connections), in order to dole out addresses to the other routers. Comcast (like most residential ISPs) tends to not take too kindly to a user putting multiple devices on their network.
Back to your question.... some people do this by daisy-chaining two routers. Sequence goes modem -> 'dirty' (AKA 'guest') router -> 'clean' (AKA 'private') router. This keeps your stuff behind the 'clean' router - anything connected to the first one (dirty) can't see anything behind the second one (clean), as they're separated by that router's firewall.
But! It's a PITA to get anything more complicated than basic internet (web & email) working behind that setup. Game? good luck. Game on Xbox Live? more good luck.
There's a better (read: easier, more secure) way. Got some, ah, obsolete PC hardware laying around? Use it - install Smoothwall Express on that, and use it as your network firewall.
Put your stuff on the Green network, and use the Purple network for the PCs you're working on. Purple is isolated (one-way - Green can see Purple; Purple cannot see Green) at the iptables level.
If you still need wireless, either configure your existing router to run in AP-Only mode (and hang that off of the LAN), or get a wireless AP.