Looking to Upgrade Motherboard/CPU

Anything below i5-8600k would not be 'pretty noticeable' upgrade. But since it is CPU/motherboard/RAM upgrade in this case, it would be serious dent in your wallet - around 500$+ depending if you have a good cooler already or need one.
An example:
PCPartPicker part list / Price breakdown by merchant

CPU: Intel - Core i5-8600K 3.6GHz 6-Core Processor ($239.99 @ Amazon)
Motherboard: ASRock - Z370M Pro4 Micro ATX LGA1151 Motherboard ($106.99 @ Newegg)
Memory: Corsair - Vengeance LPX 8GB (2 x 4GB) DDR4-3000 Memory ($109.89 @ OutletPC)
Total: $456.87
Prices include shipping, taxes, and discounts when available
Generated by PCPartPicker 2018-03-14 04:26 EDT-0400

Note that your new case housing only microATX boards somewhat limits your choices, as most good Z370 boards are full size and will not fit.
 
Oh I forgot to mention my ram. I currently have Corsair CMY16GX3M2A2400C11R Vengeance Pro 16GB (2x8GB) DDR3 2400MHz, and G.SKILL Ares Series 8GB (2 x 4GB) 240-Pin DDR3 SDRAM DDR3 1600. Ultimately I am very frustrated with my current massive case is now. (https://www.newegg.com/Product/Product.aspx?Item=N82E16811352020) You know of any smaller/lighter cases in the $40-70 range that would house my current build, with the ability to upgrade at a later date?
 
I suggest finding a nice I7-4770K or 4790K and use that as upgrade

Since you had the VIVO case in your question, I found one that supports ATX motherboard like yours 45$
VIVO ATX Mid Tower Computer Gaming PC Case w/ Glass Window, 4 Fan Ports, USB 3.0 Sale Ends in 4 Days @ newegg.com
https://www.newegg.com/Product/Product.aspx?Item=9SIA3BG6N82399&nm_mc=AFC-C8Junction&cm_mmc=AFC-C8Junction-PCPartPicker,%20LLC-_-na-_-na-_-na&cm_sp=&AID=10446076&PID=3938566&SID=

your case size problem will be driven by the fact you have a full size ATX motherboard, if you has a M-ATX motherboard like a Asus Z87 gryphon board then you could go to a M-ATX Case like you previously mentioned.

 
Running at 24 Gb of RAM? That makes move to newer CPU even less sensible, as you would need DDR4 and RAM prices are not good now. You should probably stick to what you have for now.

As for case, NZXT S340, Phanteks Eclipse P400, Phanteks Enthoo Pro, Fractal Design Focus G are nice and reasonably priced cases that will fit.
 


Just make sure the CPU heatsink is 155mm or less in height, and that case should work great. :) CASE-V10G is a fairly compact ATX tower, and we made sure when designing it that it can keep a decently powerful gaming PC cool with the stock fans.