You can try an alternative firmware like OpenWRT or DD-WRT which have an option to increase the transmission power. A powered booster would certainly help but a different antenna could be effective depending on your situation. For example, if your client is in a certain, fixed location on the other side of a wall then a directional antenna should help. Also, if you are having issues with concrete walls then you should consider lowering the transmission frequency and/or slowing down the speed (from G to B, for example) as that will allow the signal to go further and through more obstacles due to how the energy travels through the medium. Remember that the weak link might not be your Access Point but the client. The signals have to travel both ways.