in short, yes. but, you might/might not sacrifice performance. I have had two different brands, popped them in and booted up in dual channel mode. Also, I have had 2x1gb kingston 667 with 2x512gb 667 kingstin and only be able to boot that up stable in single channel mode (I really think this should have worked, but I think my mobo was the problem). bottom line is, you will probably be able to mix and match memory, but you might/might not be able to run dual channel mode.
your best bet is to run matched memory in channel "A" and matched memory in channel "B" and you will probably be able to run in dual channel, but there is no garruntee