I'm using Crucial 4 x 4GB 240-Pin DDR3 SDRAM ECC Unbuffered DDR3 1333 (PC3 10600) Server Memory Model CT51272BA1339 on my X9SCM-F-O with Xeon E1230 (ESXi 5 and unRAID + some other guests).
http://www.newegg.com/Product/Product.aspx?Item=N82E16820148382
I have started with 4GB stick adding 4GB every other week. Now I have 16GB. This MB works with 4GB, 8GB, 12GB without any issue for me.