im going to buy new parts for my pc. now my pc is around 5 years old now but can still handle the newer games... but in lower framerates and resolutions. so i checked on our local retailer and found a good deal on parts specifically the GT560Ti, FX4100 (and sli ready mobo), and i3-2120(and its non sli ready mobo).. my current specs are in the spoiler
I'd upgrade the CPU and motherboard first. With an LGA 775 dual core and a mid-range GTX 500 series card, there's going to be a bottleneck. More importantly, one that will be noticeable.
As for the FX 4100 vs i3 2120, it really depends on the games you play most often. Most games hardly utilize more than 2 threads, but games such as GTA IV and Battlefield 3 (multiplayer) really benefit from the extra threads and horsepower.
Do you plan on overclocking or running multiple GPUs? If not, go with the i3. The FX CPUs are great overclockers from what it seem, and part of their full potential comes from overclocking which will require an aftermarket HSF. With your current PSU, I wouldn't recommend overclocking the CPU or GPU, so the i3 seems like your best bet.