I am using 2GB DDR3 1333MHz & 4GB DDR3 1333MHz RAM, in BIOS I can see total 6GB RAM, but in OS I can see only 3.2GB RAM, Why?

Earlier i was using 2GB RAM but now i have added another 4GB RAM, In BIOS i can see Total RAM as 6144 MB (DDR3 1066), But in Windows XP & Ubuntu 10.10 OS i can see only 3.2 GB RAM.

2GB DDR3 RAM ADATA 1333MHz , 240 pins, Single Channel
4Gb DDR3 RAM Dolgix 1333MHz, 240 pins, Single Channel

My Motherboard is that support 2 x 8GB DDR3 RAM of 1333MHz & 1600MHz.

i also tried to interchange slots of 2GB & 4GB RAM, but it shows 3.2 in OS & 6GB in BIOS.

Motherboard is H61H2-M2,

Below is my System Information
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System Information
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Time of this report: 9/26/2017, 23:16:24
Machine name: --------
Operating System: Windows XP Professional (5.1, Build 2600) Service Pack 2 (2600.xpsp_sp2_rtm.040803-2158)
Language: English (Regional Setting: English)
System Manufacturer: ECS
System Model: H61H2-M2
BIOS: BIOS Date: 09/01/11 16:57:15 Ver: 04.06.04
Processor: Intel(R) Pentium(R) CPU G620 @ 2.60GHz (2 CPUs)
Memory: 3312MB RAM
Page File: 345MB used, 4846MB available
Windows Dir: D:\WINDOWS
DirectX Version: DirectX 9.0c (4.09.0000.0904)
DX Setup Parameters: Not found
DxDiag Version: 5.03.2600.2180 32bit Unicode