Upgrading a PC

[EvoRo]

Budget (including currency): 500-600 usd

Country: USA

Games, programs or workloads that it will be used for: gaming

Other details (existing parts lists, whether any peripherals are needed, what you're upgrading from, when you're going to buy, what resolution and refresh rate you want to play at, etc): 

I currently have:
CPU- Intel i5 6400
Mobo- MSI B150M PRO-D
GPU- MSI GTX 750 Ti
HDD- WD Blue 1 TB
RAM- 2 x 4GB DDR4 2400
PSU- 400W ATX
Monitor-  Asus VG248
Hello.  I will be upgrading my PC for the first time to make it last a couple more years until I can afford a  brand new build.  (I use my PC mainly for gaming.  It lags while multitasking and games load slowly.)  I'd be keeping the CPU and motherboard.  I'm thinking of upgrading to a GTX 1660 (S or Ti), adding a Samsung 860 EVO 1TB Internal SATA SSD, replacing RAM with (16 or 32 GB) DDR4, and possibly a higher watt PSU.   Do you have any suggestions?  What is the highest GPU I could get with this older CPU/Mobo?  Do you recommend any specific products?  The RAM the PC came with is 2400 (2x 4gb), even though the motherboard specs state 2133 MHz.  I appreciate any and all help.  Thanks so much!

[Herman Mcpootis]

a lot of these need an upgrade tbh, wait out the current shortage and then figure out what to buy later.