Memtester running on Linux is a more throughout RAM tester than the usual memtest.
Info on the max RAM speed: http://forum.notebookreview.com/threads/does-x220-i7-support-1600-1866mhz-ram.732786/
Apparently, only older/modded/coreboot BIOSes let you use 1866MHz ram that helps quite dramatically with the GPU performance.
Simili dual-channel works if you use 2GB+4GB sticks for instance. Bringing the speed benefits on 2GB of RAM (benefits the GPU as well).