OptiPlex 755 Memory problem

Hello,

      Just recently I bought 4 Gigs of ram for my OptiPlex 755 and installed them. During booting the computer stated that the memory had changed. When I opened My Computer / Properties It states 6gigs installed (only 3gigs usable). I searched around and I believe the problem is my system runs a 32X version of windows.

      What is the easiest way to fix this problem so that my computer runs using all 6gigs of its installed ram.

My computers system Information is below

Time of this report: 2/5/2014, 15:51:21
Machine name: CPU                                                                                                                                                                                                 Operating System: Windows 7 Home Premium 32-bit (6.1, Build 7601) Service Pack 1 (7601.win7sp1_gdr.130828-1532)                                     Language: English (Regional Setting: English)
System Manufacturer: Dell Inc.
System Model: OptiPlex 755                 
BIOS: Phoenix ROM BIOS PLUS Version 1.10 A21
Processor: Intel(R) Core(TM)2 Duo CPU     E8200  @ 2.66GHz (2 CPUs), ~2.7GHz
Memory: 6144MB RAM
Available OS Memory: 3070MB RAM
Page File: 1553MB used, 4584MB available
DirectX Version: DirectX 11

Graphics

Card name: NVIDIA GeForce GT 610 
Manufacturer: NVIDIA
Chip type: GeForce GT 610
DAC type: Integrated RAMDAC
Display Memory: 2257 MB
Dedicated Memory: 978 MB
Shared Memory: 1278 MB
Current Mode: 1024 x 768 (32 bit) (85Hz)
Monitor Name: Generic PnP Monitor
Monitor Model: COMPAQ FS7600