[Ask] About speed and recommendation

I'm trying to do a bruteforce SHA256 13 characters with custom char

And the mask:?1?d?d?a?a?a?a?a?a?a?a?a?a

I tested using Nvidia GeForce GT 630

I get speed 408615.5 kh / s and maximum completed in 10 years>

My question:
1 How many words per second if my speed 408615.5 kh / s
2 If I use dual Nvidia 750Ti. estimate how long the hash solve?
3 How to calculate the speed of kh / s with a gpu

I'm looking for a solution, what kind and how much is needed to accelerate vga SHA256 hash is 13 characters

So

Thank you