I say keep your store credit for something else. The CPU upgrade isn't worth it at this time. The performance increase in gaming and general use would be about 10%. And unlocking PCI3 doesn't gain you much, if anything as most cards cant even saturate a PCI2 bus. As to the RAM, the 2500k can fully utilize that, you just have to configure XMP profile in the motherboard's BIOS.
I would just stick with what you've got. PCI-e 3.0 doesn't really make much difference at all at this point. Even in situations where it gets a bit of a boost, 3-way SLi or CF, it still only gains a tiny bit of performance here and there. If anything, since it's a gaming PC first, throw another 680 in there and be set on the GPU front for the next three years or so. By that point, PCI-e 3.0 might be relevant and warrant a CPU upgrade.
Would I notice a decent speed boost for gaming and general use?
I have a gtx 680, so this upgrade would unlock pci 3, and I have some 1600mhs ram that the i5 2500k dosent use to its full speed
Any help is much appreciated
For games, No. The i7 3770K is the sweetest choice if you're gonna do video encoding, rendering or generally gonna use a lot of multi-threaded application.
In real world performance there's no difference between PCIE 3.0 and PCIE 2.0, performance is the same.
Also, In real world performance there's no difference between 1333 MHz RAM and 1600 MHz RAM, maybe a additional bunch of numbers if you ran a bench program.