I have no idea what a 420X ATX case is, but if it is an ATX size, it will probably hold almost any length card. An i5 could mean anything from Intel's 1st gen i5 quad core processors to the latest (and much faster) 8th gen 6-cores.
The 650W PSU could be anything from a cheap no-name over-rated PSU to a fine high-quality unit. But I would suspect that your only limitation will be your budget (?). The GTX 1060/1070 and AMD RX-580/Vega gfx cards would be good choices... if you have money to burn. They are all extremely over-priced right now due to the crypto-miners and the surge in Chinese gaming.