Amplifiers are available to boost transmit power, receive sensitivity, or both. The FCC limits the maximum power output of a device operating under the part 15 rules (which includes 802.11) to 1 Watt. All 1 Watt amplifiers I have seen will not produce more than a Watt out no matter what the input level is.

When selecting an amplifier, be careful to get one designed for 802.11 service. Other amplifiers on 2.4 ghz might not linearly amplify the signal across the entire band that 802.11 uses. Another factor is the TX (transmit) range vs. RX (Receive) sensitivity. There is no sense transmitting a signal farther than the transmit range of the other station. An adequate RX preamplifier helps in this situation. In a simple point to point link, having the same equipment on both ends makes sense because there is a guaranteed symmetry.

How powerful of an amplifier do you need? Probably not as much as you think. Signal strength is related to power logarithmically. This means that doubling transmit power only brings 3 dB of signal strength on the receive end. Obviously there are diminishing returns after a point. Higher gain can be had more economically by choosing better AP and client hardware, a better antenna, or better orientation of an existing antenna (both in terms of height and azimuth). In addition, an amplifier creates band pollution by sending the signal farther than it needs to be sent, and in the wrong directions. This also contributes to the security problems of 802.11. Using a large amplifier is akin to shouting across a crowded room: it's not very effective, takes a lot of effort, you annoy a lot of people, and give away information you might not want to. If the receiver cups one's ear, and the talker uses a megaphone, much less effort is required and you annoy less people.

I have used amplifiers from Hyperlink Technologies.

Some amplifiers include a built in lightening arrestor. This is very handy. Amplifiers are also available with the option of power over the coaxial cable. This is handy because there is now one less wire to run. Unfortunately, this technology comes at a premium.

Use of an amplifier and a highly directional antenna can create RF fields that are dangerous to humans. I'll post RF exposure limits here when I find them. HINT: Don't stand directly in from of a dish or yagi when it's connected to an amplifier and transmitting!