2 gtx 460's sli: 768mb with 1gb

Okay, I know it's possible to configure cards of the same model from different manufactures in sli. Would it be possible/advisable to pair a 768mb gtx 460 with a 1gb gtx 460. I was thinking about putting a 768mb in a new system now, and then throwing in a 1gb model later when prices drop a little more. Would the system scale down the 1gb card to the 768's level and refuse to recognize the extra ram? Would it work at all?