It would draw more power than the PSU can support and create a system where one wrong bump and you fry the entire system. Up to you how much risk you are willing to take, but I would not recommend doing this.
See here for specs on the GTX 660, note the power requirements.
The SFF case limits your PSU selection to TFX form factor PSUs. Not many of these are available above the 300-400 Watt range due to size restrictions and the fact that high end high power consumption cards very rarely come in low profile variants. If you could find a 400 Watt TFX form factor PSU, that would be sufficient. The other alternative is you get a full sized PSU and simply run it outside the case, but if you do that you might as well get a new case too and try to transplant all your other hardware to it.