wired connection from wireless router 2 WAP

Recently I have started doing a lot of the networking for the company I work for, but I am running into some problems I was hoping you guys could help me with. We usually have rg-6 and cat 5 in about every room running back to our control panels so its not hard for me to hard wire anything if needed, that said:
My problem: When a customer has a mansion or 3 or 4 story house and I can’t get a good enough wireless signal across the house.
basically I am trying to connect wireless routers together to broadcast the same signal. They will be wired together I am not trying to connect them wirelessly to each other. Are there any standards for doing this? Because so far I am having a hell of a time. Especially If they are two different brands.
For example I have a Linksys wrt54g and a wrt54gs at my house. It is probably pretty simple but how do I connect these two devices together? I am betting that it is easier to connect two Linksys routers then two different name brand routers. For instance I have a customer with a wrt54gs and I have an On-Q Access point. I have this access point installed in the ceiling upstairs and it is hardwired to the wireless router downstairs. I can only manage to set it up to where if the customer is downstairs he has to connect to one ssid and if he is upstairs he has to connect to the other. After decoding what I am trying to ask can someone please point me in the right direction. I am not that great at networking yet and I am not sure what all these different modes are in my router, therefore the answer could be right in front of my face and I would not know it. Thanks for any help

 

The Brand does not matter.

It is done this way, http://www.ezlan.net/router_AP.html

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Thanks alot i will give that a shot. I am on my way out so ill get back to you and tell you how it works. alos i was wondering if this is the same for the wireless access points. Becasue this is a perfect example of how to do my two linksys routers and i am hopeing it will work but is there something different about configuring my On-q access point? I have not tried messing with it yet so i may be able to figure it out. just looking for a heads up.