You won't tend to get much increase in FPS with an overclock but the frames that you do get are better somehow. Just check out what's on sale in the amazon warehouse, bargains to be had on everything but before you buy just look at the benchmark results on tests here
http://www.tomshardware.co.uk/pentium-g3258-overclocking-performance,review-32974-4.html
http://www.tomshardware.co.uk/gaming-processor-frame-rate-performance,review-32628-9.html
The g3258 is currently £55 ish right? So is the fx4300 95w. I've tried overclocking the 125w fx4350 & got to just over 4.7ghz but I'm finding heat to be a big problem at vCores necessary to push up beyond that. You need a mega heat sink, mega plus in fact.
The fx 4300 might do a bit better because it is a lower wattage cpu & quad rather than merely dual core.
http://www.tomshardware.co.uk/answers/id-3032141/gtx-950-gtx-750ti-amd.html
This thread discusses the pro's and cons of amd vs nvidia.
Then go google all your prices, from the motherboard to the memory - check out the amazon warehouse first.
Also if you get the amd fx then you should also get one of the fx990 chipset overclocking boards from asus, msi, or gigabyte and check out the reviews on those.
Do not skimp on the motherboard or the HEATSINK - I would recommend something like the coolermaster v8 if you plan to overclock. You need a heat sink that is waaaaaaaay over specced. and then some if you want a stable overclock at higher clock speeds and voltages. Plus there is an element of chance in the whole bargain - the silicon lottery. You have no way of knowing how good your cpu is until you try it.
So the only way to eliminate doubt is to get a high binned cpu like the fx8370e. The low watts on that will lend it to overclocking higher at lower vCores. Meaning less problems heatwise - but still, make sure you don't skimp on the heatsink and buy a monster.