I always try to boost the router first, since that will improve the situation for all the wireless connections. Then I look to the individual computers after that.
Hawking Tech has a number of products that will help you increase your wireless range. The root page is Hawking Hi-Gain™ WiFi Range Extending Products
Some of the more interesting products are this Hawking [HSB2] Hi-Gain WiFi Signal Booster
, which can be used on either end of a wireless connection to boost the signal power.
Another way to increase your signal strength is by the use of hi-gain antennas. You can choose from omni-directional or directional models, here are a some examples.
Hawking [HAI7SIP] Hi-Gain 7dBi Omni-Directional Antenna
Hawking [HAI15SC] Hi-Gain 15dBi Corner Antenna
[HAO14SD] Outdoor Hi-Gain 14dBi Directional Antenna Kit
This is just a sample of available products, many people have hi-gain antennas with similar specifications, but I haven't seen any other suppliers of signal boosters.