You will -not- see any real world difference between PCIE3 and PCIE2.
Upgrading your I5 2500K to IB would be worthless - as you already have a pretty powerful CPU. IB has about 7% better IPC than SB, but also uses less power.
Basically IMO it would be a big waste of money for you to upgrade now. Toms reccomends upgrading your CPU which is at least 3 tiers above your current one. IB is probably just 1 tier above the i5 SB.
If your going to upgrade - wait for haswell next year.
One more thing - depending on resolution, you probably won't notice a difference. For example - at 1920x1080 for my resolution - I can max out all my games on Ultra(my specifcations are in my sig). My setup is actually quite similar to yours.