ECS c51gm-m motherboard only recognizes 1/2 of installed RAM (memory)

Recently purchased & installed DDR2 4GB PC6400 PNY RAM on my ECS c51gm-m v1.0 motherboard. When I check the system properties it's only showing a little under 2GB. I realize that 32-bit isn't going to recognize the full 4GB because of "memory mapped IO reservations." But I expected to see at lease 3.2GB of RAM. I am running XPPro using labor intensive photo editing software & need all the RAM I can get from what I've installed. I've looked at the manual for the c51gm-m motherboard as well. It states that this motherboard can support up to 16GB RAM. I also understand that PNY isn't on their Qualified Vendor List for memory. I have used PNY in the past & had good luck with it. The manual also only shows up to a 1GB stick & the manufacturers that are supported. How can the motherboard support 16GB when they don't show up to 8GB sticks that are supported? Can anyone give any suggestions as to what to check for or any other help? It would be greatly appreciated. I only have a 15 day return policy on the RAM & if it's not going to give me at least 3GB I am returning it.

Thank you.

please see when the system boot on the post screen and in the bios whether it is showing full gb of ram. it may shared by your on board agp device or so.