Well you could go with an i3 cpu, such as the i3 2100 but even a good combo deal with a CPU and motherboard will run you about $250 or more.
Not to mention the newer 1155 motherboards all use DDR3 ram, so you would have to upgrade that as well, which would probably be another $50 for 4GB.
So it depends on what your budget is. If you are looking at staying within your 100-200 budget, a quick upgrade on the CPU might make more sense for the short term until you can save up a bit more in the future for a new build. But essentially it's just a band-aid.
However, if you are looking at increasing your budget, you might be able to go with a better system. I'm only interested in what PSU you have if you could post that too.