Yup.. They will all work at 1066 MHz.. You'll be loosing the dual channel advantage though.. But i guess you can live with it as it is not so much of a performance degradation.. Higher RAM count is anyday more beneficial..
^ Yes, they all will work at 1066. HOWEVER, the OP will not bee loosing Dual Channel. As long as the OP matches the 2x1GB and the 2x2GB in to the correct DIMM socket, it should be fine. Each set will run at Dual Channel.