Is it OK to mix DDR3 memory modules?

[diamondhindend]

First off, The computer I bought recently has four slots, two are being used by the memory modules it came with (2x4) and two are open.

I would like to add 2x4 (max is 16gb) memory modules but I'm wondering if it will harm the system because they don't match the modules the computer came with.

So, if it is OK to mix them, that brings us to my second question.

Between the two listed below, which one is better?
http://www.newegg.com/Product/Product.aspx?Item=N82E16820148544
http://www.crucial.com/store/mpartspecs.aspx?mtbpoid=73C5CD70A5CA7304
Or is there something better you could recommend?

 

[False_Dmitry_II]

I don't know if it's changed, but alot of people back in the DDR2 days would be screaming no and saying computers never work like that.

For me however, I've practically always ended up running stuff mixed in all kinds of ways.

Until my actual 2x8 set comes in, right now I'm running what was laying around because I just switched to DDR3.
I have 2x2 GB at 9-9-9-24 1.65 volts and a single 4GB stick (came with the mobo) at 9-9-9-24 1.5 volts. I'm running all of it, so the 4 gig stick isn't even dual channel.

Still runs fine. I've never had a problem with stuff being completely incompatible. In fact I only ever look bemusedly at the lists mobo makers give you as to what ram will work before ignoring it entirely.

Even if you can't get the advantage of it, because of the ram you're running more than likely at a higher volt, I'd get something low voltage.

Like this: http://www.newegg.com/Product/Product.aspx?Item=N82E16820231461

Actually if I was you, given the small gap in price right now, I'd get this:

http://www.newegg.com/Product/Product.aspx?Item=N82E16820148663

Then switch them for what you have and sell what came in your computer on ebay. RAM has gone up lately so you'd probably get 40-50 from doing it and the net you spent on these would be less.