Unfortunately, there's not a whole lot you can really "upgrade" to, because a) you already have a 2C/4T APU, which is as high on core counts as they go, b) all of the listed CPU options for your motherboard (https://www.asus.com/Motherboards/F2A85M_LE/HelpDesk_CPU/) that have higher frequencies are all at the same tier on the CPU hierarchy (http://www.tomshardware.com/reviews/cpu-hierarchy,4312.html), & c) as @Ecky pointed out, Socket FM2 is essentially "dead" (having been replaced by Socket FM2+, which is now being replaced by Socket AM4).
That being said...you have a couple of options available.
1. Overclock your current CPU. The "K" means it's unlocked, so you could get an aftermarket cooler & boost the clock rates a bit. It might give a noticeable boost, it might not, but it's at least an option.
2. Since you have a dedicated GPU, you don't necessarily need a "full" APU in there, since you're not using the integrated graphics. Although not technically an upgrade, you could do a lateral shift to one of the Socket FM2 Athlon chips. The Athlon X4 750K is from the same "Trinity" line as your current APU: slightly slower base core speed but slightly faster boost core speed, still unlocked for possible OC, & doesn't need a BIOS upgrade. If you can upgrade your BIOS to version 6102, it'll handle the "Richland" Athlon X4 760K (slightly faster than your current APU).
3. As you were thinking, RAM upgrade is crucial. I would lean towards at least the 16GB upgrade. I would also consider maxing it out (your board can take 32GB in a 2x16 configuration).
4. Depending on the software you're using, I would really consider a GPU upgrade as well. Even a GTX 750Ti, let alone a more modern GTX 1050 or one of AMD's RX series, would be a monumental upgrade. What you'll want to look for, though, is if the software you're going to use has the ability to take advantage of GPU hardware acceleration. Some applications, for example, can benefit from nVidia CUDA cores, so getting a nice GTX 1050 (or even a GTX 1060, depending on how much horsepower you need) would be beneficial.