500 or 650W is not the most important.
what you really need to look for, is the max power available on 5V rail.
something have greatly changed since the amiga and the first ATX revision, it's mostly everything moved from 5V to 12V.
nowdays, psu give the maximum on 12V.
original amiga 4000 psu is something around 90W on 5V, so you need something similar.
modern atx psu usually give 100W on 5V.
and you also have to know, as your amiga suck nothing on 12V, this can seriously unbalance modern psu regulation. and affect 5V performances.
maybe you can consider adding fictive load on 12V to prevent this. usually, it's fine without this.
but you can safely look for 400/450 modern psu. only matter the 5V, you need at least 100W (20A)
less you get on 12V, better it is for 5V and your amiga
