By 'seamless' I mean that a host can travel throughout the coverage area of the wifi, moving from one router or AP to the next without the connection dropping.

I have:
a),b) 2 routers with dd-wrt firmware installed

c) the cable gateway from Virgin media, which also acts as a router, with wired hosts attached to it

d) another router, which works but can't use dd-wrt.

e) one powerlink adapter which can connect two devices together 

I can't connect two devices by ethernet, as I can't drill holes in the walls, and two devices in the same room would not extend the range usefully.

Device (c) has to stay where it is.

Devices a) & b) have the following possible modes:

* AP
* Client
* Client Bridge (routed)
* Adhoc
* WDS station
* WDS AP

There's also an option to set up something called a 'virtual AP'. Apparently a limitation of the Atheros chipset that these routers can't be set to *repeater* or *repeater bridge* mode.

In the past I used d) to extend the range of wifi by changing the SSID to the same as c) and setting them to broadcast on different channels. I can't remember whether that was seamless or not though.

To get necessary coverage, I think I'll need at least 3 routers, if not all four. What's the best way to do this seamlessly?

edit: I want to do this with what I already have, not buy more stuff. I would like the wired hosts attached to c) to be on the same subnet as all the wireless hosts.