We are considering bringing in wireless Internet from a local ISP (802.11b) but we are concerned that our own wireless point-to-point Cisco bridge (1310) might interfere with the ISP link. Will operating on different non-overlapping channels avoid RT interference? The equipment would be about 50-100 ft apart.
Depending on what country you're in there are up to 14 channels in the 2.4GHz range for 802.11g. 802.11b uses 3 of those channels (corresponding to 1, 6 and 11 in the 802.11g range) according to SolutionBase: 802.11g vs. 802.11b. That article has an excellent discussion of b and g interoperability and some pretty good reasons why you shouldn't even support b (ie security).
Generally I try to keep them about 5 channels apart. I have run two 802.11g networks just fine at home on, say, channels 3 and 8 where the access points have been less than a foot apart with no problems. That's what the channels are for.
Channels and international compatibility is a good read actually. You'll note that the overlap is about +/- 2 channels so 5 apart is a good rule of thumb. It also lists various international differences between allowed channels.
Also worth noting is The myth of non-overlapping channels: interference measurements in IEEE 802.11
I'm assuming that by "wireless Internet from a local ISP" you mean that your LAN's connection to the Internet will be going over 802.11b to a base station some distance away that's owned by your ISP.
Many people seem happy deploying 802.11bg APs using 1, 6, and 11 as the channels. The theory is that the center frequency for each channel is 5MHz apart, but the communication channel needs 22Mhz, so you have to be 5 channels away to avoid most interference. It will probably work fine as long as the ISP connection and your 1310 are operating on different channels.
If all of the devices have highly directional antennas that are aimed properly, that will reduce the chance of interference. I assume that the local device from your ISP will have a directional antenna pointed to their base station.
You could also replace your bridge with one that uses 802.11a (5GHz) instead of 11b.
Regardless of what you do, your neighbors may have a WiFi network already that could cause interference, so that's something to keep in mind if you're thinking of relying on a wireless ISP that uses 802.11b.