Here are some ideas....it will be hard though to seat that many people without a big space.  http://www.avsforum.com/avs-vb/vbclassified.php?do=category&categoryid=41

Have you thought of a inflatable screen and a projector for outside use?  That would be your best bet to accomodate that large of a group and the plus side would be everyone would be outside....who cares if they leave there shoes on and spill the sodas.  I believe Best Buy carries inflatable screens.

http://www.bestbuy.com/site/olspage.jsp?_dyncharset=ISO-8859-1&_dynSessConf=7016224919804740131&id=pcat17071&type=page&st=inflatable+screens&sc=Global&cp=1&nrp=15&sp=&qp=&list=n&iht=y&usc=All+Categories&ks=960