Can I run 2 x TWIN2X2048-6400C4

I have 2 x1gig right now at DDR800 ,can I run 4 sticks and keep DDR800 or will I have to drop down to DDR667 ? Thanks.

If you mean on the IP35, it should run 4gb just fine. In fact, in a recent review, it did very well with 4 x 1gb. Just make sure you match it with the EXACT same RAM so you are not mixing different types/ICs etc.