ATX PSU questions?

I am in the process of upgrading my cartesian style printer from knock off ramps board to a Duet WiFi.I have a cheap power supply that came with the printer and it seems to work pretty well but I am considering the ATX for simplicity and the ability to turn the printer on and off remotely with the simple use of a wire.I had a few questions on what size ATX would be good and whether or not a full modular ATX would be a better option for wiring or will the connections need toi be made internally?The reason I ask about the connections needing to be done internally is because of the size of the supplied wiring?I am running a single hot end and a 300 x 200 heated bed.I am looking at somewhere between 450 - 650 watts and a reputable company like EVGA or Corsair.

I used a 450W Corsair modular PSU for a while to power one of my printers. It worked well, better than the cheaper ATX PSU that had been supplied with the kit.

Most ATX PSUs need a dummy load on the main 5V output in order to work properly.

For anything larger than a basic printer with 220x220mm bed, I recommend using 24V power.

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Large delta printer [miscsolutions.wordpress.com], E3D tool changer, Robotdigg SCARA printer, Crane Quad and Ormerod

Disclosure: I design Duet electronics and work on RepRapFirmware, [duet3d.com].

Instead of an ATX power supply, let me recommend an HP DPS-600PB server power supply. This is a 47A 12V power supply that can be bought for around $20 shipped on eBay, doesn't require a dummy load to give full power, can be switched on/off with a single wire like an ATX supply, can be easily modified to allow two in series for 24V, and is pretty much bulletproof. The only thing I don't like about it is that even when it's "off" the fan still runs to ensure that the circuitry that provides +5 standby power is kept cool - in my case it's not a big deal because I unplug the power supply when the printer is not in use, but it's something to take into account.
I use a pair of them to power some battery chargers for my RC hobby, so when it came time to upgrade the power supply on my A8 it was a no-brainer to use a DPS-600PB.