In the older WDS method, you can configure an AirPort as a relay between a main and remote base station. This can still be accomplished with the 802.11n AirPorts, but it comes with a hefty bandwidth penalty loss ... and the overall extended wireless network can only operate in the 802.11g radio mode. As such, I wouldn't recommend it in your case.
The best option, but not easily implemented if you don't already have an Ethernet backbone running throughout your home, is a roaming network.
Thanks for your suggestion.
My problem is I can't use an cabled ethernet connection between my Time Capsule and my Airpot Express on the 2nd floor which is presently configured as a relay.
My second Airport Express on the 1st floor captures the Wifi signal from the one on the 2nd floor and I can use its Ethernet lan connection from a switch. My problem is that as it is not in direct wifi link with my time capsule it will not allow wifi clients to join it...
I was wondreing if swapping the Airpot Express acting as a relay by an Airport Extreme would solve my issue?
OK - I will get to using an Extreme later but you could first try this to use what you already have - use the Express on the second floor as a wireless extender of the TC - it would be broadcasting a wireless signal on the second floor and it's ethernet jack would be active - you would then run an ethernet cable to the Express on the first floor and set it up to "create" a wireless network with the same name and password and security type as the TC.
If you wanted to get a new Extreme, I would set it up as your primary base station in place of the TC (because it is more powerful) and use the TC on the second floor in place of the Express and use the Express on the first floor as above.
However, as Tesserax said, the best way to do it is this:
New Extreme on 4th floor - hardwired to TC on 3rd floor - hardwired to Express on 2nd floor - hardwired to Express on 2st floor - they would then create a true roaming network on each floor.
Let me know how you make out.
Thanks again for your suggestion.
I wanted to avoid using hardwiring between floors whih is why I had opted initially for the CPLs. Unfortunately for some reason I can't figure out, the connection between the CPLs had become completally unreliable and I had to give them up.
I guess I'll live with my setup as it is now. The only problem with it being the weakness of the signal on the first floor.