I would suggest a more powerful device to start with, most commercial models will totally solve your problem , look at www.zigwire.net
Product Name:AID-2405sg
Product Class:Long Range Wireless LAN system
Features:
* 5 levels of transmit power adjustment ,* Indication of 'connection status,* 5-dbi super rubber antenna, 500mW/27dBm output

This has 5 times the power of what you are using now and will work well. or this outside unit even do a much better job.
Product Name:AMU-2405G-SMA
Product Class:Long Range Wireless LAN system
Features: AMU-2405G-SMA 802.11g/b 500mW , AP/Client/Repeater/Bridge; LED Wan/Lan/Power; Lighting arrestors protection; Stainless Steel adjustable mounting bracket;No internal antenna panel; Plastic outdoor weatherproof case; -30c~65c ,120/240~48vdc power ,up to 30km with Yagi, 16-dbi gain[not included]