Would help to know where you will be purchasing as it affects pricing and availability. Any preferred online vendors and a budget would be a help too. It is kind of hard to pick without knowing this and usage (overclocking, hours of use, etc.) Here are four suggestions in the budget price range with varying wattage.
https://pcpartpicker.com/products/compare/873RsY,zNK7YJ,DPCwrH,nqs8TW/
The FX 4300 is a tier 4 CPU and for AM3+ you would likely have to move to a FX-8xxx series CPU to see a good upgrade.
http://www.tomshardware.com/reviews/cpu-hierarchy,4312.html
But your mobo is very low quality budget board and I caution installing an FX-8xxx CPU.
http://www.tomshardware.com/forum/id-2384030/motherboard-tier-list-am3-chipsets.html
But if you are trying to squeeze every bit of life out of the build without fear of system failure I would be looking at an FX-8300. Stay away from any 125W CPU's.
https://pcpartpicker.com/product/qmrcCJ/amd-cpu-fd8300wmhkbox
You might want to consider a new CPU/Mobo/RAM a Ryzen 5 1400 build or i3-7100 build would be a great budget choices depending on which games you play. The 1400 is an overclock capable build.
PCPartPicker part list:
https://pcpartpicker.com/list/VF8zRG
Price breakdown by merchant: https://pcpartpicker.com/list/VF8zRG/by_merchant/
CPU: AMD - Ryzen 5 1400 3.2GHz Quad-Core Processor ($157.49 @ SuperBiiz)
Motherboard: Gigabyte - GA-AB350M-D3H Micro ATX AM4 Motherboard ($84.49 @ SuperBiiz)
Memory: G.Skill - Ripjaws 4 series 8GB (2 x 4GB) DDR4-2400 Memory ($59.92 @ Amazon)
Total: $301.90
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-12 08:28 EDT-0400
PCPartPicker part list:
https://pcpartpicker.com/list/TrnqXH
Price breakdown by merchant: https://pcpartpicker.com/list/TrnqXH/by_merchant/
CPU: Intel - Core i3-7100 3.9GHz Dual-Core Processor ($109.59 @ SuperBiiz)
Motherboard: MSI - B250M PRO-VD Micro ATX LGA1151 Motherboard ($63.88 @ OutletPC)
Memory: G.Skill - Ripjaws 4 series 8GB (2 x 4GB) DDR4-2400 Memory ($59.92 @ Amazon)
Total: $233.39
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2017-07-12 08:35 EDT-0400