What's the purpose of the upgrade? That is to say, for what reason are you upgrading? How much are you willing to spend? It really depends on what you want to achieve with the upgrade.
On that particular spec I would personally upgrade the CPU to an i5-4570 or similar (but that's for my own usage). There's little advantage of a 'k' processor as the motherboard isn't designed to overclock, so you can save a little with non-'k' processors. MSI's
site shows the CPUs compatible with your motherboard.
An SSD would decrease boot times considerably if you don't already have one. Depending on what you use your PC for and how much you already have, additional RAM may be worth considering.
Graphics card... I would say it depends on the games (if you play them) and the CPU. Depending on your power supply, you may even need to upgrade that to use some of the more powerful graphics cards.