Are you trying to add ram or replace? You also may have a bad stick. 32-bit will not use all the RAM, but 64-bit may hit problem areas.
I also googled your error and it may be video card related. Boot in safe mode, uninstall driver, and see if the problem persists in normal boot after that.
The ram is a strait across replace. atm i have 1g 1g 512mb 512mb pc2-5300 and i would be swaping 2g 2g 2g 2g pc2-6400. I have tested all the drivers and have narowed it down to the ram... i can run my old ram in windows 7 fine but when i try and use 8gigs i just get the BSOD...
Download memtest86, and test ram: http://www.memtest.org . Before you do that, make sure your ram has appropriate settings in the BIOS like proper timings and voltage. You may try and relax timings in the BIOS (increase their values). These are CAS, RAS, etc.
Yes, you have the AMD SB600 South Bridge. This chip has issues with 64-bit addressing mode using more than 4GB RAM. The solution is either a BIOS update or you'll have to limit the RAM to 4GB. Here are two topics about it on Anandtech Forums but similar reports have been discussed on ASUS Forums as well.
Further to what the other people posted check to see that the motherboard provides enough volts for the new ram sticks. Usually faster memory requires slightly higher voltage compared to slower memory. This could be causing you the STOP messages. Also, just for information, Win7 has it own memory test tool. You can access this by going to the Control Panel -> Administrative Tools -> Windows Memory Diagnostic. Hope this will help.