Is this the BIOS only reporting 4GB after POSTing? It could be a memory seating issue. Is this a board with 3 or 6 DIMM slots? Try to remove and replace the modules in the 3 slots you have them in... and if its a 6 slot board, try the other set of 3. Inspect the 'original' slots for a bent pin or some damage that may not allow the memory to connect properly.
The motherboard manual will tell you which slots are recommended for triple channel operation... which you'll want to make sure is active once the system recognizes all 6GB. Good luck!
I was going to get to that after finding out if 4GB was reported after post, or 6GB
I remember reading that Microsoft changed Windows to report 4GB when 4GB was installed, even though the system could only really use 3 to 3.5GB. I guess system builders were getting customer complaints that their 4GB system (with a 32-bit OS) only had 3GB in it!