Mix Ram speeds

Hi
CPU-Z says I have an Intel DQ35MP motherboard & in research I see it can go up to 800mhz

For some time, I have been using 2x 1gb DDR2 PC-9600 Patriot Extreme 1200mhz RAM - which clearly has only been running at a max of 800mhz

So today I bought 2x 2gb PC2-6400 @ 800mhz & they work fine

However, my Windows experience rating under RAM has gone down from 5.9 to 4.6, yes its 1200mhz back to 800mhz but its 4gb v 2gb & the motherboard was 800mhz anyway

So the question

1 still have 2 slots free, can I put back my 2x 1gb 1200 RAM to work aloingsde the 2x2gb 800mhz RAM ?

If so, slot 1 & 3, should I have the 2x 1gb 1200 or 2x2gb 800mhz ram ?

Motheboard says it can take 4x2gb ram & theres 4 slots

Thanks in advance