I use the CM Hyper 212 evo cooler, which is brilliant imo.
My GPU is MSI GTX 770 4gb.
As for overclocking the CPU, I am scared to do it, never fiddled with the BIOS before and I would be worried I would fry it or something.
If you've got the 212 I'd look into over locking your 2500K. It's fairly easy and you should be able to take it to around 4.5GHz, maybe more.
You could look into upgrading your GPU. It's where you'd get your biggest jump. You'd get around a 30% jump or so. Another area is an SSD if you haven't already got one.
Outside of those areas there's not much else worth spending money on.