reformat has nothing to do with memory. Thats referring to your hard disk and no you don't need to reformat anything to add memory.
You say you have two sticks now? The key question is are there more empty memory slots to add memory into? Take a peek and see. If you have unused slots for memory then yes you can add more.
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You say you have two sticks now? The key question is are there more empty memory slots to add memory into? Take a peek and see. If you have unused slots for memory then yes you can add more.