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).