At first glance, the first likely candidate for upgrade is your CPU; what you should get would depend in part on your motherboard. For example, you wouldn't need an unlocked CPU if you have a H61 chipset. You might then increase your RAM to 8GB.
That was at first glance. Menetlaus makes a good point though, we need to know what you want to do with this PC. What doesn't run as well as you'd like? You have a decent graphics card, so if something isn't running well, once again I'd suspect the CPU is the weak link.