Installed additional 1GB of RAM but Windows does not detect it

I have recently tried installing a 1GB module of Samsung DDR2-800 (PC2-6400 [400 MHz]) RAM alongside the existing 1GB Kingston module of DDR2-533 RAM (PC2-4300 [266 MHz]) that my laptop already had. Although my motherboard supports only up to DDR2-533 RAM speeds, I have tried installing the Samsung RAM module by itself and a few other DDR2-800 RAM on my laptop and they worked fine (although they will run at DDR2-533 speeds). I had also tried installing a 2GB DDR2-800 RAM module (2nd slot was blank) previously and Windows was able to detect all of the 2GB memory, but I had to transfer that RAM module to another laptop.

I believe that I should have a total of 2GB RAM but Windows XP reports my total RAM as 896 MB (128 MB as shared video card memory). My BIOS also reports the same as Windows. On the other hand, CPU-Z detects all of the total memory that I have installed (see CPU-Z details below).

Specs: Laptop is an ASUS A6R, Intel Celeron M 1.7 GHZ, 2 RAM module slots, Windows XP.

I checked out my laptop's memory upgrade info on crucial.com here: http://www.crucial.com/store/listparts.aspx?model=A6R
According to this link, this laptop can support a maximum total memory of 2GB(2048MB) and that 1GB memory modules are supported. Is there any workaround that I can do to make Windows detect all of the memory I have installed? Anything that I might have installed or configured incorrectly?

Edit:
I also tried swapping the places of the RAM modules in the slots, but the issue persists. I also tried installing the Samsung module by itself (2nd slot is blank) - Windows loads fine with this RAM module.

Also - weird: If I install the 1GB Samsung RAM by itself , the BIOS and Windows reports 960 MB total of available memory, much larger than the 896 MB if both are installed. Does anyone know the reason for this?

CPU-Z details:

Chipset
————————————————————————-

Northbridge ATI RS400/RC400/RC410 rev. 01
Southbridge ATI SB400 rev. 00
Memory Type DDR2
Memory Size 2048 MBytes
Channels Single
Memory Frequency 266.5 MHz ()
CAS# latency (CL) 4.0
RAS# to CAS# delay (tRCD) 4
RAS# Precharge (tRP) 4
Cycle Time (tRAS) 12

Memory SPD
————————————————————————-

DIMM # 1
SMBus address 0x50
Memory type DDR2
Module format SO-DIMM
Manufacturer (ID) Kingston (7F98000000000000)
Size 1024 MBytes
Max bandwidth PC2-4300 (266 MHz)
Part number 9905295-031.B00LF
Serial number AC0E6761
Manufacturing date Week 20/Year 07
Number of banks 2
Data width 64 bits
Correction None
Nominal Voltage 1.80 Volts
EPP no
XMP no
JEDEC timings table CL-tRCD-tRP-tRAS-tRC @ frequency
JEDEC #1 3.0-3-3-9-12 @ 200 MHz
JEDEC #2 4.0-4-4-12-16 @ 266 MHz
JEDEC #3 5.0-4-4-12-16 @ 266 MHz

DIMM # 2
SMBus address 0x52
Memory type DDR2
Module format SO-DIMM
Manufacturer (ID) Samsung (CE00000000000000)
Size 1024 MBytes
Max bandwidth PC2-6400 (400 MHz)
Part number M4 70T2864QZ3-CF7
Serial number 76F36D66
Manufacturing date Week 03/Year 10
Number of banks 2
Data width 64 bits
Correction None
Nominal Voltage 1.80 Volts
EPP no
XMP no
JEDEC timings table CL-tRCD-tRP-tRAS-tRC @ frequency
JEDEC #1 4.0-4-4-12-16 @ 266 MHz
JEDEC #2 5.0-5-5-15-20 @ 333 MHz
JEDEC #3 6.0-6-6-18-24 @ 400 MHz

I don't have a detailed explanation for you, but perhaps the most important feature to consider is that you are dealing with DDR2. Therefore, you should use matched sets when using 2 sticks of RAM. No need to buy the high-prices exact match set from the same production run, but the specifications should be exactly the same.