What's the budget? New or used? If the 400w psu is good quality and not a piece of junk (Gold has nothing to do with quality) the best you could do is a gtx1660ti, but that's brand new, best used would be a gtx1060 6Gb (2 steps down from the 1660ti) but equitable to a gtx970 (about 4 steps up from a 750ti. Avoid AMD gpus, you don't have enough power to use anything that'll be better than the gpu you have.
I like to upgrade my gpu to gtx 1660 ti. Then what psu should i go with? Can you recommend me a brand?I'm sorry. You are at the limit of what that psu can do. Only just. Power Logic psus (they make that psu) do not tell the truth about anything. According to the sticker, that's 20A and 18A on 2x 12v rails. According to everything I could find, there's only 1 rail controlled with a transistor that's rated at 350w generally, 400w if you get lucky. My Seasonic 520w is 40A by comparison. So I'm not understanding how that adds up to a 400w unit having almost as much power as a 520w Seasonic. Not with both being group regulated.
Power Logic also uses some of the worst, cheap, low quality capacitors there is, and other low quality components on the pcb. It's a running joke amongst many pros as to how much power it'll take before it explodes. Not IF, but WHEN.
Again, sorry, but I'd not advise putting a more powerful gpu on that psu