The big question here is: Will the cpu bottleneck the 650ti, and the answer is yes, in some games the E6800 will bottleneck a 650ti, and in some games the 650ti will be the limiting factor.
So is it worth the extra $$?
Tbh i don't know, it really depends on what games you play, if you play a lot of cpu heavy games then i would go for the 650ti, if you play a lot of gpu intensive games then i would go for the 750ti.
If you want to, you can tell us what games you play then i'll give you my opinion on which one you should get.
-Hope this answers your question.