What is the best cpu this motherboard can handle?

Solution
Any 64bit processor can handle that much and even much, much, much more than that.
http://www.howtogeek.com/175443/what-is-the-maximum-amount-of-ram-you-could-theoretically-put-in-a-64-bit-computer/
It's up to MB and it's BIOS to determine RAM size limit. A 32bit OS can also address only about 3.5GB of RAM.
List of compatible processors is in that link of yours. Q9xxx, socket 775 are most capable processors in that list.
Any 64bit processor can handle that much and even much, much, much more than that.
http://www.howtogeek.com/175443/what-is-the-maximum-amount-of-ram-you-could-theoretically-put-in-a-64-bit-computer/
It's up to MB and it's BIOS to determine RAM size limit. A 32bit OS can also address only about 3.5GB of RAM.
List of compatible processors is in that link of yours. Q9xxx, socket 775 are most capable processors in that list.
 
Solution