How much two-way communication do you need? Which computers need to see the others on the network, or do they all need to see each other? I think the cleanest way would be to have the wired router upstream of the airport -- then the two wired computers are already visible to the iBook because they have unique upstream IP addresses.
Your setup sounds right -- your wired router has a WAN port and four LAN ports, right? And your AEBS is connected to one, with your two desktops on two others of the four, right? That is basically correct.
Did you try running airport setup assistant after plugging it in that way?
The only tricky thing I can think of is that you should probably not have both routers use the same virtual addressing -- that is, if one of them assigns addresses off of 192.168.x.x, the other should use 10.0.x.x. Otherwise, it could get confusing, because for the downstream router, the assigned IP address upstream will look like a downstream address that it thinks it should be assigning....
I had a Linksys wired router with two wired computers on it and an AEBS with my iBook configured this way, though, and I did not have trouble, so I'm sure it's *possible*.
