IMac :: Use 1066 Ddr3 In 2010?

I ha e 2x2gb of 1066 ddr3 ram sitting here. I am going to buy an iMac. I can't decide between the refurb 27 core2duo, the refurb 27 core i5 or the new 21.5" i3. Of course I am torn between cost and performance. I would prefer the bigger display but it's not necessary. If I could use my existing ram in the new 21.5 that may put it as the winner but I am unsure if I can mix 1066 and 1333 ram or not. If not then I will probably get the refurb 27 and then it's just to decide between i5 or core2duo