I would first try to grab an amplifier for your antenna. You can put a big antenna on your computer, but if your wireless card is under powered to begin with, it won't make much of a difference.



You can try this if you wish to keep your current wireless card (prices are pretty decent on eBay):

Amazon.com: Hawking HSB2 HiGain Signal Booster: Electronics



Try this if you don't mind changing cards (it's a USB wireless card and 600mW amplifier in one):


Amazon.com: Amped Wireless High Power Wireless-300N 600mW USB Adapter (UA600): Electronics



If you have a window that faces the general direction of where the WiFi is being transmitted from, you could use this antenna in conjunction with either of the items I listed above.

Amazon.com: Hawking HiGain Directional 12dBi Window Antenna - HA12W: Electronics



As for using the china-made omni-directional antennas, I would avoid them. I've tried them out myself, and performance with them is rather disappointing. A good rule of thumb is to stick with name brand antennas. They might be more expensive, but they offer the performance they claim (most of the time).