i5 and i7 both have 4 physical cores.

i7 has hyper threading allowing it to allocate 2 execute threads per core thus windows sees it as 8 cores. It still has only 4 physical cores.

Hyper threading gives you extra performance depending on type of operations used with allowing the core use idle cycles with another (the second) execute thread.

However this does not give you twice the computing power compared to non-hyperthreading processor as it just tries to optimize the usage of those 4 cores with extra threads.
As normally you're wasting clockcycles to non-cpu operations and the extra threads are supposed to be utilizing these cycles.

How much extra it'll give is hard to say. depends so much on the operations run.
My wild guess is average of 20-25% more than similar architecture and clockspeed non-hyperthreading processor. You'll get some figures by seaching for i7 reviews.