Very nicely put, eibgrad. That would work quite well.
Option 2: Smoothwall. Hang your '655 off of the Green LAN interface, and the '634M off of the Purple interface. Purple is completely isolated from Green (one-way isolation - Green can access Purple, but not vice-versa), but still has full access to the internet.
The main computer that needs to acess the Internet is my desktop. The HTPC does not. What if I put two NIC cards into my desktop. Can I have one plugged into each router so I can access both routers from the same machine, but not have the two routers plugged into eachother? Is it possible then for someone to gain access to the Wireless N router (through my desktop)?
1- Are you sure that your HTPC needs zero access to the 'net? Media Center (were you using the term loosely, or referring to Windows Media Center?) especially really needs 'net access - Guide updates, MC app updates, movie data.....
2- Multi-homing your desktop (what you want to do with two NICs there) is a Very Bad Idea, especially with your 'public' WLAN being completely unprotected. Big security risk, not very hard for someone to break down that door.
Oh, and, I trust that you've disabled "wireless admin access" on the 'public' router, yes?