3400G + 1660 Super?

If it's a good 500w power supply then it will work. If you have a Corsair, Seasonic, or EVGA power supply then you can count on it. If your power supply unit is of a brand that no one has ever heard of, or you paid $30 for it then don't count on it. Otherwise, a good quality 450w power supply would be enough to power a Ryzen 3400g and a GTX 1660 Super. It's not like these 2 parts use a ton of power.

If you plan to upgrade to something like an RTX 2080 or RTX 3080 in the future then you will need to upgrade the power supply.

The Ryzen 5 3400g is about the same as the OEM only Ryzen 5 2500x. The 3200g and 3400g are made with the preceding Generation's Zen+ architecture that was used in Ryzen 2000 series processors. The reason the 3400g is priced as high as it is, is because it come with integrated graphics. If you don't need the integrated graphics then Processors like the Ryzen 5 2600, Ryzen 3 3100 or 3300x and the Ryzen 5 3600 are better choices if you can find them for about the same price. 

The 3400g wont bottleneck the 1660 Super. My problem with the 3400g is it's not a good value for a gaming rig. I know people HATE Intel these days but something like the Core i5-10400f or Core i5 11400f are way better gaming processors than the 3400g.

search internet for psu sizer app [website].  input your cpu and gpu, plus other specs.  [i use the one linked at lower left on newegg's power supplies page]