Wireless Network Help

OK got a brand new G5 tower, dual 1.8, etc etc. Airport express card inside the lovely metal box . DLINK 614+ (dual antenna). Mac works fine if I leave it in the living room next to the router. Upstairs Bout 40' away diagnally through ceiling, it can't see a thing. PC laptop (compaq evo) has 4/5 signal strength and works fine. Bought buffalo WLA-54G accesspoint/bridge with 4port ethernet for the extra ports (internet phone, etc). But since the DLINK and buffalo don't want to talk to each other, and DLINK doesnt do WDS with Buffalo, that doesnt work. I've heard i can buy an antenna for the MAC, but reviews are REAL sketchy....any ideas, comments?