Does RAM brand really matter?

It does matter, unfortunately. But before I go down that route, going to Staples is going to be a complete RIP OFF!   Only because they charge more for the good stuff than they should. Buy from Newegg, amazon or alike. MUCH cheaper for better quality, and you're only going to wait an extra day or two (presuming you're asking around on forums, you can wait the extra day or so, to save $20.  

Brand. Yes, it matters. They break down into 3 tiers, but unfortunately for flaming reasons I'm not going to list them like that. Just know that tier 3 is the worst (the cheapest) and anything goes, even if they have a lifetime warranty. Tier 2, middle of the road price as you might expect, but you struggle with installation/DOA modules. If it starts ok, as well you get past the 30-day mark, you're in the clear for the lifetime of the system (most of the time).
Tier 1 is only a tad more expensive than tier 2, but you have the best probability against DOA and the 30-day window failure. As well you'll have MUCH better support should the *** hit the fan.
Given that RAM is the most likely device/component in a new build to fail, this is saying something.  

With RAM being so cheap, go with Kingston, Corsair, or Crucial.

As for exactly what type (speed/etc) ram to buy, it's hard to say because you haven't posted what system you have. Ideally you match exactly what you have now for inter-compatibility reasons. But if your upgrade is going to require you remove the existing stuff then, we really need to know what your system/motherboard is to help you further.

CPUz is a good tool to ID your existing RAM part numbers as well as your motherboard part number (as long as it's not an OEM system, like a Dell, HP, etc - if the latter, make and model of your system will do).