The ASUS M4N68T-M - http://www.asus.com/Motherboards/AMD_AM3/M4N68TM/#specifications
only has (2) DIMM slots, so IF you have (1) 4GB stick then get the EXACT same 4GB stick. Generally, it's best to purchase a set of 2x4GB and with your CPU DDR3-1333 2x4GB set.
RAM is cheap, and 64-bit games do benefit from >4GB of RAM; nice read -> http://www.tomshardware.com/reviews/ram-memory-upgrade,2778-8.html
; note the 64-bit game on the bottom