Antec makes good units, so there's probably no need to check voltages. It's
not hard to do, though, if you can borrow a multimeter. Just be sure to avoid
touching high voltage points!
It sounds more like a hardware/driver conflict, or possibly bad memory. Maybe
check the BIOS setup for anything related to AGP memory, or possibly
conflicting IRQ assignments for the PCI and AGP cards.
With AGP there's a block of main memory reserved for graphics use - called the
AGP aperture or something similar. A conflict might come about if the drivers
for the PCI cards aren't well-behaved and try to use that block at the same
time as the AGP card.
You could remove the AGP card and try the two x1300s together ... I'm not sure
if you tried all possible combinations.
Maybe try the x1600 with a non-ATI PCI card too.
Good Luck!