Ideally you will want a separate USB Powered HUB with its own power adapter, in your case an ATX power supply. This is a good power supply for 3v,5v and 12v as its clean and does not suffer from voltage droppage on loads and peaks.
Make sure to use the 5volt (RED) cables only on the Pi and ideally
keep the phase separate, meaning power the Pi from one cable and the
HUB from another cable strand.
If they are simple VGA 2D cards only they will usually work of the Pi but as mentioned before you don't want to overload the Pi. 3D devices will have separate power connections. Theoretically you should not use more than 2.5Watts per USB port off the Pi (5v * 0.5A)