The density of RAM dimms (the amount of ram per stick) your system can support depends on your motherboard. What is the model number of your system (I'm assuming it's a laptop)? You should be able to find out exactly what your system supports using Crucial.com's "Advisor Tool".
http://www.crucial.com/usa/en/memory-info?cm_re=top-nav-_-flyout-memory-_-us-memoryI use it alot just to get the specs of what my system supports and then I'll go wherever I want to actually buy it (typically, I use newegg).
I just finished reading the last paragraph of your post (I tend to stop halfway through sometimes and answer before I'm fully informed) and saw the "Mac only" comments. I'm not sure of the context I said that in (and I hope it was correct for the context) but as far as what components are used to make the ram stick, that's true. Macs use Intel hardware so the memory chips in a Mac dimm are the same as memory chips in a "PC/Windows/Linux/Blah" mem stick and as long as the specs of the memory are supported, and it can physically fit in your system, it should work.
Now, laptops are a special breed. Windows-based laptops (Dell, Asus, Toshiba, etc) typically use a standard form-factor for their ram dimms (so-dimm is the form factor). I haven't ever had to work on a Mac (old or recent) so have no clue whether they conform to this form factor or not. If so, you should be good. But if your laptop model uses a proprietary form factor, then that might be the "Mac only" aspect of the advertisement.
Here's the wiki page to the "SO-DIMM" form factor typically used in laptops today.
https://en.wikipedia.org/wiki/SO-DIMMchrist is a far better source of information on this (as well as a plethora of other subjects) so helpfully this will tide you over until he's able to grace us with his presence.
Get it? "christ"...."grace"...
