A lot of photoshop rendering and MAYA Rendering's.
What's the price gap between 16gb of DDR3 and DDR4 and what are the differences? Also I might invest into 32gb of ram if I need to.
No. Absolutely not. 32 gb of ram is overkill.
o-v-e-r-k-i-l-l
If you want to go DDR4, you will be set for the future since its ability to access the hard drive and to give info to the GPU and the CPU is nominal comapared to DDR3. But, again, you have to check the compatibility with your motherboard/stuffs like that. DDR4 is moreso for server farms.
You'd rather go for DDR3 though for the sakes of money.