Patriot Viper 3 4GB PC3-12800 DDR3 Desktop Memory for $26.99 shipped after a $10 rebate.  Here is a screenshot before the $10 rebate for your reference.  For the technically inclined, this dual channel RAM operates with 10-10-10-27 timing at 1.5V, which makes it compatible with most standard systems.  More information is available here.