That costs 2-3x what a 2008 Mac Pro 3,1 is worth & is complete overkill for the OP's requirements.
BTW I know that your said that you don't want to mess around with flashing BIOS but a used GTX680 that you can flash yourself to a Mac edition gives you an Apple boot screen plus support from native OS X drivers. It's the fastest card with native drivers & a boot screen. Faster cards are not useful for the 3,1 as the CPU will be the bottleneck in the system not the graphics card.
yeah, I know that. However, I was just answering his question without any extra personal opinion.
Anyway, it seems that I was fail to give him the best answer (even not considering the cost).
He don't want to flash the card by himself. So, only 680 Mac Edition, 7950 Mac Edition, and MVC card, these 3 options available. (It's too risky to get flashed card from other source, so, not recommend to OP).
He don't want to mess around any system files. So, 7950 + 3,1 combination is out of equation due to
possible power management bug which require mod the system file to fix it.
In fact, I should assume him also don't want to mess with the web driver as well. So, any card beyond the 680 (e.g. Maxwell card) is also out of equation.
And now, the 680 Mac Edition (new card) should be the best card. However, it's almost impossible to get one now. So, MVC 680 should be the best choice.
However, if including the $200-300 budget requirement, the MVC 680 is also out of equation as well. So, the in budget best card become MVC 760.
http://www.macvidcards.com/store/p20/Nvidia_GTX_760_2_GB.html
N.B. I know there are plenty of other better choice to get a better card with lower cost, or give OP better gaming experience. However, I am just answering his question.