My home is CAT-6 hardwired. We have a Quest Modem with 4 outlets on it for ethernet and unlimited wireless. We needed more hardwire outlets and I purchased a Linksys N router with both capabilities (wire and wireless.) I thought I could plug the Linksys into the Quest modem and it would be capable. My son's XBOX 360 drags when plugged into it? I have a weather station plugged in and it works fine. Obviously I realize the difference in demand, but why? Is piggybacking not an option? I can spend more money and buy a wireless attachment for the XBOX but didn't think I would need to.
Wireless is usually about half the speed of wired even with good wireless reception. You should try to optimise your reception and avoid interference.
1) raise the router above furniture level
2) Experiment with channels (some will work better or worse depending on your environment)
3) If you can detect strong neighbouring wifi, use a channel 5 stops away from strongest.
4) Relocate cordless phone base or video sender etc.
5) Be prepared to move the computer (or at least turn it so your body is not between the router signal and the wireless adapter's antenna.
It sounds to me like the quest modem has a built in router. This will cause problems when equipment connected to your quest modem attempts to communicate with things connected to your linksys.
What is the model of your linksys router? Your best bet would probably be to setup the linksys as a WAP instead of a router. To do this, you will need special firmware such as DD-WRT. Right now, both your quest modem and linksys router are trying to assign IP addresses and handle network traffic. This can be a big problem if they are both assigning IP addresses in the same scope/range (ie 192.168.1.x 255.255.255.0). The easiest solution is to return your linksys router, and purchase a simple switch. This will allow you to add more ports to the quest modem and still use the built in wireless. If you prefer to use the linksys router, it is best to disable the wireless on your quest modem so that it doesn't interfere with your linksys and then configure your linksys as a WAP so that the quest modem is the only device handing out IP's.