670 leaves plenty of room....???? and than when time comes for upgrade is going to be better that he sell again...because 670sli leaves nothing later!!!
if you want power now do sli....
This is just poor reasoning, but I guess with that username I should have expected as much.
OP you will be happy with both options, though each has its drawbacks. SLI 570 will give you the best performance, but when the time comes to upgrade to something new you will be replacing both cards and will have a tough time selling them. Single 670 is an "easier" setup and will overclock to stock 680 specs. When you want to upgrade from the 670 you have the option of 2x or 3x SLI and you wont be trying to sell two very outdated cards. If you get a reference model 670 you will have some extra room in your setup as well, but of course you are going to be paying $70 more than the 570.