Power supply: if you intend to get the RX 460, then 500W would suffice. The RX 460 reportedly requires 380. From memory, it may be a good idea to replace the power supply if you can afford to with a better quality one.
BIOS: to be clear I'm not saying you have a locked BIOS, I've even linked threads whose owners reported success in upgrading. I am saying a locked BIOS is possible with pre-built PCs. If a pre-built PC has a locked BIOS then is there a way to upgrade? In a manner of speaking, yes. But it would mean replacing the motherboard and at that point it might be time to consider a new PC entirely.
My advice of making sure you can easily return a graphics card if you do have a locked BIOS still stands. Otherwise you'd end up with...