RAM Upgrades: What should I get?

Will this setup actually run smoothly? The eBay seller told me I shouldn't "mix" them—though, honestly, I don't see any real difference here...
thanks

http://www.ebay.com/itm/RAMAXEL-4GB-...3D222585133461

My current RAM:
https://postimg.org/gallery/1kh1p88ru/

I remember once picking up some RAM at a local shop, and even though the speed and all the other specs matched perfectly with my existing 4GB stick, they just wouldn't play nice together. I ended up grabbing a different model the very next day, and that worked like a charm. My point is—you’re probably just going to have to take a bit of a gamble here. Though, I have to ask, where on earth did you find all those pages on eBay? Where are you even located, and did you bother checking Craigslist at all?

It’s because the RAM modules are the exact same brand as mine—same MPN and everything else—plus they’re pretty cheap. Though, I suppose I’d happily shell out $33 more just to have total peace of mind that everything will run smoothly...

When you're picking out RAM, you can't just look at the capacity; the frequency (MHz) has to match up too. And seriously, watch those motherboard slots—they aren't all created equal, and things won't always work if you just plug them in anywhere. If you’ve got four slots and one stick is already sitting in the first slot, there’s a good chance the second one won't play nice in the very next spot. You might have to skip a slot and try the third one instead.
On my setup, for instance, I have four slots available. I already had an 8 GB dual kit occupying slots 1 and 3, but when I tried adding a new dual kit into slots 2 and 4, my PC just refused to boot. I ended up having to pull everything out and reorganize the whole thing—I put the first kit in slots 1 and 2, and then the second kit went right into slots 3 and 4.
As for voltage, it's usually not a huge deal. My system runs just fine with a mix of 1.35V and 1.5V paired together.

At that price, once you factor in the customs fees, you’re basically paying the same amount we do here. I'd probably just hit up a reputable shop and check if they have a return policy in case the fit is off. Or honestly, maybe just go into the store, test out a few modules on the spot, and grab the ones that actually work.

By the way, there's a limit to how high you can crank that frequency. He’ll be able to support whatever speed he's currently running at, but you absolutely cannot drop the frequency below what's specifically rated for your RAM

wearycrane15 said:By the way, there's a limit to how high you can crank that frequency. He’ll be able to support whatever speed he's currently running at, but you absolutely cannot drop the frequency below what's specifically rated for your RAM

Why wouldn't I want my RAM running at a lower frequency than what's printed on the stick?