Can I use 800fsb 2gb & 667 fsb 1gb together in my gigabyte mother board?

I have build a desktop myself with AMD Phenom II X2 550 with gigabyte 78 mother board. the question is I have purchased a new ram of transcend 800 fsb 2 gb and I have an older ram which is also transcend 1gb but it is only 667 fsb can I use them together in my pc? pls give me suggetion