Is upgrading my CPU worth it??

Upgrade definitely worth it. You current cpu is weaker than old Intel Ivy-Bridge cpu, the Ryzen are equitable to Intel Haswells. About 40% stronger performance in gaming ability.

With a fixed budget, a stronger cpu outweighs the minor difference in ram size. 8gb is a minimum recommendation, you can run windows and just about every game on 8Gb, especially at 1080p. If you were wanting a top end build with crossfire or sli gpus on a 4k monitor, the yes 16Gb would be advisable, but for your pc 8Gb is fine, most games won't use that much, you might see 50-70% usage at best.

2 threads in 2 cores is much stronger than 2 threads in 1 core. Knowing this, a 1500x is 4c/8t or basically a Haswell i7, like the older i7-4770k. The 1600 has 6c/12t. So in games like Witcher 3 or GTA:V that can and do make use of 8 threads, a 1500x is topped out with the 4 core strength splitting to 8 threads. The 1600 not only brings a full 4 cores alone, but only 2 cores are split into 4 threads. This can be huge in fps difference when the core bandwidth is saturated. It's close to having an i5-4670k plus an extra i3 helping. Worth the difference? Yes, many times over.

Ram is an easy swap, takes seconds and there's no guess work, no mess, no wondering if you got the paste on right or the possibility of bending a pin or 3. Once seated, a cpu is best left alone, it's not the easiest swap there is. Most will replace an entire build and never once have upgraded the cpu. New ram by comparison is child's play. And relatively inexpensive overall.

As to the mobo, that's a personal decision. I own an MSI mobo, love it. It has 4x ram slots and 2x8Gb ram. If I was to add ram, I'd buy a whole new kit, either 4x8Gb or 2x16Gb. I'd not just add more. The chips on the stick all come from the same batch of silicon. As do the chips on the other stick. Silicon isn't pure, it does have impurities. The ram itself has more than just the 5 numbers of primary timings you can usually see, like 9-9-9-27 1T. There's also secondary and tertiary timings, over 30 in all and all those have to be compatible, as well as the impurities of the silicon, or you have issues. Sometimes it's a simple voltage bump, sometimes system agent needs adjusting, sometimes the ram only runs at 1 speed slower or very relaxed timings, if it works at all. I've had 2 sticks of identical ram, from different batches, same vendor, timings, speeds, voltage, even color was exact, only difference was the batch number. They worked great alone, but would not work together. At all. I've had ram of total difference work great together. There's simply no guarantee at all. So even if you decided to get the mobo with 4 slots, later added another 8Gb of ram, there's no telling if it'll work. If it doesn't, you send it back and hope the next replacements do. Again they might not. Crap shoot. So best bet is alway buy a new kit. Factory tested, guaranteed to work. You want 16Gb? You buy 16Gb, sell old 8Gb on eBay. So a 2x ram slot mobo with 32Gb capability is just fine. So is a 4slot 64Gb capability. Thats not a question. The only question should be does it have what I want. Audio, Sata, USB, m.2, colors, leds, whatever.