Dell Optiplex 755 desktop - What is the Maximum amount of memory I can install?

I have a Dell OptiPlex 755 with 2 Gig RAM.  What is the maximum I can install?  I can't seem to find this information on the darned Dell website!!! Here's what I know about my pc:
223-0731 : OptiPlex 755 Desktop Energy Smart,Core 2 Duo E6550/2.33GHz 4M,VT,1333FSB
330-1129 : Shipping Material for System Single Hard Drive Neo Desktop Dell OptiPlex
365-0257 : CFI Routing SKU
366-1551 : Custom Project Fee for ESLH
313-5426 : 16X DVD+/-RW,SATA,Dell OptiPlex 755 Desktop or Minitower
310-9498 : vPro Secure Advanced Hardware Enabled Systems Management Dell OptiPlex
 
420-6972 : Windows XP PRO SP2 with Windows Vista Business License Dell OptiPlex,English,Factory Install

366-1040 : Custom Asset Report
 
310-9627 : Dell USB 2 Button Optical Mouse with Scroll,Black OptiPlex
 
374-5789 : CFI,B3RB,Display,Flat Panel Display,19,1908T,Black,19 Inch,VIS,Factory Install
 
420-7963 : Roxio Creator Dell Edidion,9.0 Dell OptiPlex
 
374-0812 : CFI,Information,OPTI 755 ONLY, Factory Install
 
310-8010 : Dell USB Keyboard,No Hot Keys English,Black,OptiPlex
 
341-5472 : 80GB SATA 3.0Gb/s and 8MB Data Burst Cache,Dell OptiPlex 755

311-7440 : 2GB,Non-ECC,667MHz DDR2,2X1GB OptiPlex 740
 
Part Number  

 Quantity  

 Description  
WK007

2

DUAL IN-LINE MEMORY MODULE, 1G, 667M, 128X64, 8, 240, 1RX8
366-1416 : Image Load
 
366-1556 : Custom BIOS Setting Fee
 
420-9179 : Cyberlink Power DVD 8.1,with Media,Dell OptiPlex/Precision
 
310-9490 : You have chosen a vPro System
 
341-4072 : No Floppy Drive with Optical Enhanced Filler Panel,Dell OptiPlex Desktop
 
310-9161 : Vista Premium Downgrade Relationship Desktop
 
313-3351 : Internal Chassis Speaker Option,Dell OptiPlex Desktop