I have a Linksys WRT54GS v4
the router is upstairs and the PS3 downstairs the signal strength was 80% good but not 100% so I bought a Netgear WAG 102 access point and put that in the same room as the PS3, I already had an RJ45 cable running from upstairs to downstairs so no problem there, that improved thing enormously and I got 100% signal strength from PS3 media renderer and laptops.
I then heard of a third party firmware for the Linksys WRT54GS V4 router called DD-WRT V 23 SP2, this adds a lot of extra functionality to the router including a wirless output adjustment mode, I installed this firmware and increased the wirless output from 28mw to 70mw, theoretically you can increase it to 250mw, but 70 is the recommended, I now have 100% from the router in its original location, result, the acess point can now go, the firmware has also fixed the UPNP not available message I was getting I now have UPNP available and NAT 2 without having to forward any ports, there is a lot more in the firmware that i havent explored yet, but it looks really good, so if you have a WRT54GS V4 I would highly recommend upgrading to this firmware, especially as its free.....