memory - Maximum RAM restrictions

I have a eMachines ET1862 with an Intel i3-550 and 2GB DDR3 RAM. Since 2GB is wayyy too slow (quick look at task manger shows it's been running at ~80% RAM since boot) I want to upgrade the RAM.

Concerning the maximum amount of memory I can fit, I looked up the model of PC and found it could handle up to 4GB. The websites I looked at also said it had 2 RAM slots.

Then I opened up the case and found 1 stick of 2GB DDR3, along with three empty RAM slots. After looking up the motherboard (IT It said H57H-AM2 in a load of places so I figured that was the name) and it came up with four slots and 16GB max.

So now I'm confused: what limits the amount of RAM you can have on a motherboard? I know the OS is fine as I've got win7-64bit. Is there anything more than the motherboard and the OS influences this limit?

Answer

The Maximum amount of memory is also determined by OS, in your case depending of your Windows 7 Version:

Home Basic     : 8GB
Home Premium   : 16GB
Professional   : 192GB
Ultimate       : 192GB

Corporate      : 192GB

Comparison Chart Windows 7

Also is good to check what the brand says.

About ET1862 Here is the datasheet it says:

Maximum 4 GB

And is determined also for CPU:

Here is the datasheet of your CPU it says:

Maximum 16 GB

for your Motherboard Specifications too it says:

Maximum 16 GB

Here is a link for yours: