Issue: The signal strength at a client far away from my wireless router is bad. Can I adjust the transmit power?
Answer: Yes, but cranking the power to the maximum won't help you any.
Background-1: You might transmit farther but the noise level will be higher (and will probably bleed into the neighbouring channels; that looks like this then) and your recieve sensitivity won't be improved any, limiting your distance. If you want better range, go buy better antennae.
Background-2: Most wifi cards do not support high transmit power and high transmission speeds. See data sheets.