If you are planning to oc ram using xmp profiles there is a chance it will not work and manual oc and tuning is needed . If you plan to run it stock there shouln`t be any problems .Just remove old sticks and run memtest on new sticks to make sure they are not faulty. This way you can avoid any file corruption BSODs in future.
Just get whatever RAM is cheapest and throw it into the extra slots, though getting a single stick of 8gb would be more preferable for any further future upgrades.