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.