Originally Posted by ticket
Yes, I replaced the original Dell PSU with a brand new Corsair 400W unit. According to the Corsair spec, it can deliver up to 30amps on a 12 volt rail, and 20 amps on the 3.3 and 5 volt rails. Was a bugger to fit - I had to hack away some of the rear chassis.
The PNY 8400GS PCI card takes all its power from the PCI connector - there is no other power supply connector on the card.
It could be that the motherboard PCI connector doesn't like to handle the power being consumed by the 8400. When the card goes into graphics mode, maybe the sudden increase in power consumption is too much. The PSU can handle it, but maybe the motherboard can't. Pure speculation from me.
It could be motherboard issue, years back my dual CPU ASUS P2BDS had the same issue, AGP voltage would drop significantly with a GeForce in slot, it would hover around 3.18V and any load would make the system reboot. Next time get a 500W PSU, enough leeway for peripherals. I have the same card, actually a 8400GS running on a Intel motherboard with quad core and using a Antec 550W. So far absolutely no issues.