Gradual Component Upgrade Question

[DrwHem]

I am going to need to change out my motherboard, RAM, and CPU at some point over the next year but I cannot afford to do it all at once. My ideal solution would be to buy a component every few months and then hopefully use it on my existing setup until i am done.
My original plan was to upgrade the RAM first because that is generally the cheapest and easiest to install. I am now leaning towards DDR4 RAM and wouldnt be able to plug that into my current motherboard. Now i am thinking that the motherboard would be easier to replace but i dont know. If i got a motherboard that could support ddr4 would i be able to use my existing ddr3 ram and components?
My current parts:
Motherboard: Gigabyte 970A-UD3P
Ram: Corsair DDR3 8 gigs
CPU: AMD 8 core 3.10 GHz

I am just looking for the biggest bang for my $ with the new components. Its a gaming PC and most of my games are CPU intensive as opposed to graphically. I already have an R9 380 GPU with 8 gigs of ram. I will also need to double my RAM in the future so looking for 16 Gigs total.

I am currently looking at these parts:

CPU: AMD Ryzen 5 1400 3.2GHz Quad-Core Processor
Motherboard: MSI B350M GAMING PRO Micro ATX AM4 Motherboard or ASUS Prime B350-Plus
Memory: Corsair Vengeance 16 Gigs DDR4

Would the new processor be a significant upgrade on my current 8 core? its only .1 GHz more. I tend to not overclock anything.

[mmettin]

The best bet would be to buy everything at one time, as prices vary quite frequently.

If you did piecemeal the system, you might run into a situation where you would be unable to return items (if they happened to be defective) to the retailer and have to deal with RMA with the manufacturer.

[tyrchlis]

The days of cross generation ram slots on a motherboard are over. Pretty sure nobody makes a motherboard with support for both DDR3 and DDR4 on it.