I think the upgrade options will definitely expand once the new lineup drops. The next Gen Nvidia cards will be even more efficient so you may wind up being able to run a 660ti (or 760ti...not sure if the Gen names have been confirmed yet) with a 30 to 40% power increase and less power draw. I am excited to see what they bring to the table in the coming months. Who knows man the GTX 670 (770) may very well fit the power requirements as well. Wouldn't that be great!
Trusting that there is a Ti model...Yeah man the new 670 should be about equal to a GTX580. I would think so anyway. The GTX 570 was around 15% faster than the GTX 480. They should continue on this line of performance growth. Hopefully it's power consumption is pretty low. The 660ti should have low enough power consumption to run in the X51 though and it will definitely be faster than the 570.