The two cards would be able to SLI; however, the 1gb 460 will only utilize 768mb as that is the max of the second card. In the end, the sli will be running with 768mb and at the lower clockspeed of the two.
Now there's a catch. The current motherboard you have isn't compatible with SLI and so that'd require you to purchase a new mb and any other parts required for SLI. Given this case, it wouldn't be worth while to attempt to SLI to two cards given that they are on their way out now a days in terms of performance for 1080p. What I would recommend if you wanted to upgrade your gpu, would be to purchase an AMD card as your mb is compatible with xfire. Now if you were willing to change the mb to an SLI compatible mb, you would open your upgrade to Nvidia gpus. Hope this helps.