I have a MaxPower USB 802.11N adaptor, was about $50, and used it on a friend's iMac because the internal one didn't quite have the 'reach'. So it might work, they're stronger than the airport cards, probably because they're outside the case.
You can get them from
www.macsales.com for sure. The only thing is that since it isn't seen as an airport, you have to use it's own little software app to connect. Not an issue at all if you're setting up a network and it won't change, just set it to connect at login. But not quite as elegant as an internal airport card.
Also, you can get an external airport antenna that will connect to the Mac to boost it's range. I had one for my G5 and it helped quite a bit, again, all it really did was put the antenna outside the case.
Good old Apple, go figure. On my 8-Core, the bluetooth isn't even strong enough to run a Magic Mouse, because the BlueTooth antenna is INSIDE the case, and it can't even get the 4 feet needed to run the mouse reliably.
Sometimes I just can't stand Apple's dumb decisions.
But a USB adaptor might solve your problems just fine.