What kind of performance do you need?
If you need more performance for rendering or other multithreaded apps, then the extra cores of the FX-6300 would be good. But... you will need better than an average motherboard to support a fx-6.
Exactly what is the motherboard? CPU-Z will tell you.
2. If you want better everyday snappiness in desktop and browsing, then a ssd is a very good upgrade.
3. If you want better gaming performance then you need to find out if you are cpu limited(the FX-4100 has weak cores)
Or, if you need better graphics.
Here is how to find out:
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To help clarify your CPU/GPU options, run these two tests:
a) Run YOUR games, but lower your resolution and eye candy.
If your FPS increases, it indicates that your cpu is strong enough to drive a better graphics configuration.
If your FPS stays the same, you are likely more cpu limited.
b) Limit your cpu, either by reducing the OC, or, in windows power management, limit the maximum cpu% to something like 70%.
Go to control panel/power options/change plan settings/change advanced power settings/processor power management/maximum processor state/
This will simulate what a lack of cpu power will do.
Conversely what a 30% improvement in core speed might do.
You should also experiment with removing one core. You can do this in the windows msconfig boot advanced options option. set the number of processors to less than you have.
This will tell you how sensitive your games are to the benefits of many cores.
If your FPS drops significantly, it is an indicator that your cpu is the limiting factor, and a cpu upgrade is in order.
It is possible that both tests are positive, indicating that you have a well balanced system, and both cpu and gpu need to be upgraded to get better gaming FPS.
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If cpu core speed is the limiter, there is not much you can usefully do except try to overclock the FX-4100.
If graphics is the limiter, then a GTX750ti will be a modest boost, and a GTX950 would be better.