I just built a new system with these components:
Athenatech A449SL.400 Silver / Blue Steel ATX Mid Tower Computer Case 400W Power Supply
EVGA 122-CK-NF66-T1 LGA 775 NVIDIA nForce 650i Ultra ATX Intel Motherboard
Intel Core 2 Duo E6420 Conroe 2.13GHz LGA 775 Processor
MSI NX8600GTS-T2D256E-OC GeForce 8600GTS 256MB GDDR3 PCI Express x16 Over Clock Edition HDCP Video Card
SUPER TALENT 2GB (2 x 1GB) 240-Pin DDR2 SDRAM DDR2 800 Dual Channel Kit Desktop Memory

While assembling it, I forgot to put the spacers in between the motherboard and case and the first time I powered it on, I smelled and saw smoke coming from a tiny chip on the motherboard. I quickly shut it off, realized my error, and put the spacers in where they belonged.

The computer was then able to start, but would frequently and randomly power off. It once stayed on long enough for me to install Windows. The random power offs continued though. Despite the motherboard problem I had, I suspected the cheap bundled power supply might be the culprit, so I went and purchased a Thermaltake 430W PSU. It seemed at first that this had fixed the problem - I was even able to play Half Life 2 for an hour or so. Then the power offs started again. The CPU temp never gets above 45 or so.

So having pretty much thrown out the PSU or heating issues as the problem, it seems like the only thing left is the suspicious motherboard that I had shorted. However, I would have expected a bad motherboard to prevent booting at all, rather than boot fine but experience random power offs. Anyone have an idea what might be going on here?

Okay, try to run your system with one memory stick attached. Check what will happen.

Are you using sata or ide boot hard drive?

Its the PSU get a 500w +

your wattage on your power supply is to low for what your system requires. get at least a 500w power supply and you should be fine.

It's been an IDE drive that I borrowed from an external enclosure. I ordered an SATA one with the rest of the components and it should be arriving today.

I'll try one stick of RAM. What would that affect?

Hmm, I somehow thought 430W would be sufficient. Do the random power off symptoms that I'm getting make sense if it's due to an insufficient power supply? I wouldn't have expected the system to stay powered for upwards of an hour or two if that was the case.

i have a radeon x1950 graphics card (256mb, pcix 16) and had the same issue as you. I checked the manual and it says i needed a 450w+ psu (i had a 400w before) so i upgradded to a 500w and it works fine now.

The only reason it was browning out was:

a)when it was on for a while it got hotter meaning a higher resistance and a lower power output
b) when under load (playing a game for example) it would brown out at a particuarly graphically intensive part

It didnt brown out at all with the gfx card removed.

Ah, I see. Thanks so much for the information and explanation, jbennet! I'm going to go return this 430W and pick up something a little more powerful.

I purchased a 575W Logisys power supply from a local computer store. With it hooked up, I get a green motherboard LED turned on. Hitting the power switch does nothing except turn on a different orange LED on the motherboard. There is no case fan, CPU fan, or PSU fan activity.

Maybe it's a bad PSU. With the old 430W one, it would turn on but before hitting the power switch the green LED on the motherboard would be blinking. I'm trying to decide if I should exchange the PSU for a new one and try that or just send the motherboard back and start over.