08-16-2017 07:34 AM - edited 08-16-2017 11:10 AM
DISCLAIMER: not a Crucial employee.
HP a6700y desktop's motherboard is ECS MCP61PM-HM. It has 4 slots and supports up to 8GB of DDR2 (PC2-5300 or PC2-6400).
You currently have 4GB PC2-6400 installed. But I'm not sure whether it's 2 sticks 2GB each or 4 sticks 1GB each. Let's hope for the former. You should use Crucial system scan or just open the case to see slots. If your OS is Windows 10, you can also open task manager (Ctrl+Shift+Esc) and switch to Performance tab, then Memory tab. At the bottom there should be information about used slots.
If it's 2 x 2GB, buy the same kit and have 8GB. If it's 4 x 1GB, I would go this way: buy 4 x 2GB, remove all installed RAM and install new.
Please also wait for Crucial staff reply, though it may take some time.
08-17-2017 03:16 PM
@Wildbil7 selecadm is correct about the max for your system and your current amount of RAM. If you are hoping to add to existing RAM it would be best to run our system scanner in order to determine how your current RAM is installed. I would also note that depending on your operating system your system may not be able to utilize more than 3 to 3.5GB of RAM. You will want to ensure that you have a 64 bit operating system prior to installing. Available upgrades for your system can be found here.
Please let us know if you have any other questions or concerns.