If the system uses half of my 2 GB memory, if I buy a 4 GB memory, will it use 2 GB's instead?
If you are using Windows, no. What it uses is what it uses, and with the exclusion of Vista the OS itself along with most desktop apps don't actually consume much RAM.
The advantage of having large quantities of RAM will make itself most apparent in applications that can make use of it, such as newer, system-intensive games, CAD software, etc.
A few games that make use of vast amounts of RAM off the top of my head include Battlefield 2, Battlefield 2142, F.E.A.R, et cetera.
Also note that if you are using a 32bit edition of Windows, it's impossible for you to make use of that full 4GB of RAM.
Unless you have a pressing need for 4GB, 2GB will do you fine.
Theoretically speaking, if I had 8 GB RAM, all usable, could I turn off page file?
The BEST write-up on the subject I have found:
i got vista 64bit and with only the desktop running i lose about 30% of my 4GB ram
VISTA IS A RAM HOG