Nothing AMD has that will slot into an AM3+ motherboard is worth the money to upgrade to at this time. Since all of the cores on any higher part you could upgrade to are identical in design to the cores you have now, all you can do is add more cores and possibly higher clock speeds.
Your best value will be in spending your budget toward a change to Intel or waiting for AMD to release Ryzen, which is expected within the next three months.
In the mean time, there should be little preventing you from playing around with overclocking your current 4300 and seeing what you can get from it.