Hi, the new Apple models both express and extreme are just fine, but ensure you set them in AP mode rather than routing if you already have a gateway/router in the network, which you have already indicated you have.
Just a note to say that today hooked up a pair of TP Link Powerline units.
Much to my pleasant surprise, they work very well. I have connected the Mac Mini to one of them 4 rooms away from the router using the ethernet port and it works just fine - faster and more reliable so far than the wi fi has ever been. It also means that streaming music to my Zeppelin Air is less affected by heavy wifi use from other computers etc.
I'll probably buy another and connect it to a wireless access point somewhere around the middle of the house.
They cost around $85 for a pair which seems good value for the benefits.
Not that this analogy will mean a lot unless you grew up in Britain in the 70's (unless it aired here too), but I felt like Catweazle when he was first shown 'electrickery' when these things actually worked!!
Your wireless will be poor with concrete walls. If your starting with 3mbps already and then tring to use a wifi unit thru a wall it loses quite alot of strength, thru 2 concrete walls it will be very weak. You could try putting the Wireless AP in the roof at a central place. I do that in my 3 level home in Ak, Mine is on the 2nd level in a central position and I get it good signal over most of the house except in the last metre of the furthest corner.