Ok, I get where you are confused.
Same thing happened when I recently upgraded RAM (old ddr400 junk) in my Dell. The Dell site said that it supported 2GB of memory. However, every memory dealer (Crucial, OCZ, Kingston, etc.) said that it supported 4GB. And then again, the same thing happened with an HP I just upgraded. So I think what happens is that the companies put "2GB (or whatever size)" maximum meaning PER slot--you double that to get the total.
Go to this sites below and find your model and whatnot and see what they recommend. Better to get information from more than one source before deciding...
Crucial (my personal favorite laptop memory brand...it's what I put in ALL of my client's laptops)
Keep in mind, you don't have to get these brands or purchase directly from them, you are just checking limitations.
Newegg.com usually has the best prices on most everything.
Intel Core i7-2600K @ 3.4GHz || GIGABYTE GA-Z68X-UD3P-B3 || Corsair Vengeance 16GB DDR3-1600 || EVGA GeForce GTX 480 || Samsung EVO 256GB SSD
my feet get cold and that gives me diarrhea sometimes - Smart_Guy