Extend the range of an Airport Extreme

Run iStumbler from your Mac. (Get it here). Look at the other wireless networks in your vicinity - and check what channel(s) they're running on. You want your device to be as far away from those channels as possible. There is a 3-channel overlap. That means, a device on channel 1 will interfere (albeit only slightly,) with a device on channel 3 (or 4? I forget.) So if lots of devices are on channel 6 (most come out of the box on this channel,) you want to be on 9/10/11 or 3/2/1. I almost always just use 1 or 11.

There is also a "use network robustness" option which I think increases range but decreases bandwidth. See this Apple Article.

what you want to do is setup your express to be a "bridge" for your airport extreme network, you want to create a "new" wireless network (but if you use the same settings as your extreme, most macs should seamlessly work and switch as required)

make sure your airport express is configured not to do any NAT or give out IP's as that is maybe what is conflicting with your network currently, also make sure you give it an IP in the same subnet as your airport extreme and also that it does not conflict with any other machines