RAM halved ...

Guys, my first specific hardware configuration:

Processor: AMD X2 4200@2.100Mhz
MotherBoard: AM2NF6G-VSTA
RAM :1GB Corsair DDR2 DUAL 400Mhz (2 pieces), DDR2 DUAL 2GB Kingston 400Mhz (2 pieces)

Today I bought the Kingston 4Gb of RAM for my computer...The motherboard , the BIOS and check everything ok ... but it was showing 6GB installed!
Load the Windows XP all content, control from the control panel e. ... 3GB

What happens to my win?

CPUz control and indeed he sees 6GB .... Even all the applications i use it see 6,is something happened to Win?

Can you be little more ellaborative about which one are you useing a 32-OS or 64bit OS?

Win the belief is a 32bit but carries the words "physical extension" ... confusing something?

But it sure?

I know that the limit depends on the 32bit RAM used to define the guidelines and 2 ^ 32 = 4Giga
I knew that the limit for 32bit was 4Gb, not 3 ...