I realize this is an old thread, but in case anybody else (like me) comes across this thread while trying to do something similar -- after trying a similar setup with client mode to allow my media center devices (xbox, satellite box, etc) to plug into a local wireless router which then relayed traffic to my main internet connected wireless router, I found that I could not get traffic to properly get routed between the two networks -- for instance, while my xbox could happily communicate with the outside internet, it could not talk to my home PC which was hanging off my main wireless router. I'm no expert but after reading up on this for a while I came to the conclusion this may be by design, and could not resolve it using client mode.

Instead, I flashed both of my routers (both Linksys WRT54G's, one a v6 and one a v8) with appropriate versions of the DD-WRT firmware (took 5-10 minutes per device) and then configured WDS following the WDS page instructions on the DD-WRT wiki. This worked like a charm and took very little effort. Client mode has its advantages, but if you run into issues routing traffic between your two networks you may find WDS to be a better fit. The main drawback is that it does require both devices to support WDS, and many do not. I found DD-WRT to be very easy to install and configure for this though.