Intel CPU speeds are now ~4.2Mhz ish but they're little improved otherwise since Sandy/Ivy, your CPU could be pushed to 4.0, maybe more if there was a real need for it (I could push my CPU to 3.4/3.85Mhz+ Turbo but there's no need to,) unless all your games are CPU limited (usually simulators)
For the cost of upgrading RAM/'board/CPU, you'd see little performance gain but a new, better GPU would drive games much better on that 2560 x 1440 monitor.
Don't neglect what I said about the PSU; extra power draw (from overclocked or new CPU or better GPU) might cause poorer quality PSUs to become flaky, esp. if it's more than ~3 years old. Better to buy a more modern Tier 1 or 2 PSU of 550W-650W (enough for almost all modern overclocked single CPU/single GPU PCs now) than have a stressed PSU kill your drives or other hardware.