jmiliz:

Low to Moderate volume. The back patio is 30ft x 25 ft. I have neighbors behind me so i wont go crazy. The more beer I drink though the louder it will get. I was thinking something like a sony 200w 2Ch stereo. Since I would only have two stereo imputs, sirius and airtunes (itunes streaming) I was wondering if I could get a small amp?

I would think that a 100 watt/channel amp should be plenty.  I personally wouldn't spend too much money on it since it's lifetime could very well be shortened by the humidity in the air.  

How about something like this?

http://cgi.ebay.com/HARMAN-KARDON-HK6600-Integrated-Amplifier-HK-6600-90W_W0QQitemZ230517519404QQcategoryZ3280QQcmdZViewItemQQ_trksidZp3286.m7QQ_trkparmsZalgo%3DLVI%26itu%3DUCI%26otn%3D1%26po%3DLVI%26ps%3D63%26clkid%3D5989483300903909742