The amount of difference you will see in the overclockability of the SC version vs reference is going to be small. The real reason to go with the SC version is because the SC versions usually have a slightly better PCB and heat syncs. The answer would still be to fork up the extra 20.
Graphics vendors are wise to graphics overclocking.
The bin(or select) the best chips and factory overclock them so they can sell them at a premium.
You may or may not be successful doing it yourself. Who knows, you might get lucky.
In the case of the EVGA FTW GTX670, you get a very good OC.
You also get what looks to me like a better cooler direct exhaust exit grille.
Ehhh, screw the binned talk. I bought a EVGA GTX 670 reference and OC'ed it myself to 680 speeds easily using Precision X software. Solid as a rock! I've always thought, why pay extra to have the manufacture OC my card when I can do it myself. Just my 2 cents! On another note, if your not comfortable overclocking yourself, then pay the extra $20 and get the EVGA FTW.
If your curious and if this helps your decision making since they were talking about tempatures in previous responses, under load my card stays in the mid to upper 60's while OC'ed using the stock cooler. Depending on your cooling setup in your case and on your GPU, your temps can vary greatly. I play games like Battlefield 3, Diablo 3, and Skyrim on maxed settings in 1080p. Just know that regardless if you purchase a stock 670 versus a OC'ed 670 it'll run any game maxed out easily with no difference to you on a single monitor. Perhaps you can use this info to make a more informed decision.