If you want better gaming performance (FPS) then upgrade the i3 to an i5. If you want better load times in both gaming and opening programs etc. then go with an SSD. SSDs make your whole computer feel lots faster, but won't really help with FPS.
While upgrading to the 2nd generation Intel® Core™ processors like the Intel Core I5 2500K would give you some great performance increases. Along with the processor you would have up upgrade the motherboard and maybe even the ram if it is using a ram that is over 1.5 volts. This would raise your total cost over $400 US. The simple fact is that upgrading to an SSD right now would give you the most visible performance increase. Adding an SSD like the Intel SSD 320 80GB would lower both boot times and access times for any program located on the drive.