From my personal experience, 2 WIRE routers don't give the best signal, and you cant enhance it with store bought antennas
Well, most likely, it is ether a bad card or a cheap antenna, which one, I don't know. This is a little more costly than what you have, but it will serve you much better than what you have now for sure. This is the combination I use in my backup computer (both bought from frys):
PCI adapter:::
FRYS.com*|*D-Link
Antenna:::
FRYS.com*|*Airlink
The D-link adapter is a little more expensive than your current card, but is worth the extra money, especailly if your trying to avoid the problem your having now... and D-link tends to be a little more reliable, in my opinion.
The antenna from Airlink101 is a 10 dBi, omni directional antenna. I get 5 bars all the time, through 4 dry walls, 50 feet away. Just don't buy other airlink products. Again from personal experience, they tend to not work right. This antenna though, does work, very well.
Also I must add, I must be getting all five bars because I am using a D-link DGL-4300 gaming router. It gives me 5 bars on all my comps, including a comp that is even further than 50 feet away. So what I'm trying to say, is that you may need to buy a better router, if the new PCI adapter or Antenna don't work for you. So if you ever think about buying another router, I very highly recommend the D-link DGL-4300. Even if your not a gamer (that i know of), you will see a drastic improvement in signal strength and coverage.
The only other router or adapter that will give you better speed and coverage is a wireless N product. D-link, I again, recommend for the job. But I would wait for a couple of years before getting wireless N products, they are only draft.
I don't know if any of this helped you, but I hope it did
Borgslayer