Note: Depending on the games you are trying to play - and the settings - you will find that a $750 budget may not suffice to achieve 144 FPS to saturate your monitor. You may want to wait a little bit longer to have a larger budget. eBay is a great place to find your parts for cheaper.
- A high quality PSU is rated for at least the amount written. A low quality PSU is rated for at most the amount written. This is why everyone above is trying to sell you a different power supply. EVGA and Seasonic are great brands, I'd recommend at least a 700W power supply for your system. This is because you always want to leave more headroom than you need, as the power supply will be most efficient and safe when not under full load.
- I'd recommend at least the Ryzen 7 2700X if you want to saturate 144 FPS in most games.
- The Vega 56 is more than powerful enough for most games at 144 FPS.
- The memory you have picked out is fast - and will compliment your system well.
- I'd recommend a better SSD. Western Digital and Samsung are my top picks. I would find a budget M.2 so you can make use of that slot. Just make sure if you do it's not a SATA M.2 drive.
- You should get yourself a HDD to store your games as well. A 1 TB Western Digital HDD usually goes for around $30 on eBay.
This is just my analysis of the situation. Hope this helps.