Make everything RAM!!!! nope, just kidding. Actually HDDs and caches exist for a reason. The reason being cost efficiency. A single ram stick with good software covers all general purpose scenarios. read more
or you can have a 4gb RAM module in one bank and 2 gb RAM module in other bank.But you sholdn't use a 4gb RAM module and a 2 gb RAM module in one bank. as you say , it's not a good idea to have unequal amount of ram in a bank. read more