The Nvidia GeForce 8800GTX graphics card, requires using TWO 6-pin PCI Express power cables,http://www.playtool.com/pages/psuconnectors/connectors.html#pciexpress
MINIMUM power requirement for using TWO 8800GTX's is;
1) Using one 8800GTX requires a 450 Watt power supply.
Must have a 12 Volt power rail capable of 30 Amp's.
(If more than one 12 Volt power rail, add the amperage of all, to arrive at the total Amp's)
Using TWO in an SLI mode requires 750 Watts minimum, and around 50 Amps.
Let me ask a couple of questions;
1) Does your Power Supply have the minimum required Wattage, and Amperage on the 12 Volt power rail? (Or rails)
2) Are you using a Power Supply, that has FOUR 6-pin PCI Express power cables?http://www.playtool.com/pages/psuconnectors/connectors.html#pciexpress
3) Looking at the adapter power cable, shown on the right side of the above link;
IF, your Power Supply does Not, have the recommended amount of 6-pin PCI Express power cables; did you use -> TWO 4-pin Peripheral power cables, for -> EACH adapter cable?http://www.playtool.com/pages/psuconnectors/connectors.html#peripheral
(Mistakenly referred to as a 'Molex' power cable. Molex was the first to design the connector. Name kind of stuck, like calling a adjustable open-end wrench, a Crescent wrench)
Why the line of questioning?
Computer unplugged from power, Anti-Static Precautions FOLLOWED, remove the power cables from the cards.
A) Is the connection end of either power cable burnt? (There should be FOUR cables) Look close.
B) Remove the cards from the PCI Express x16 slots.
Look down into the PCI Express x16 slots at the contact pins. (With a light)
How about the gold plated contact pins on the cards?
There HAS to be TWO 6-pin PCI Express power cables, for EACH card.
There HAS to be TWO 4-pin Peripheral power cables used for EACH 6-pin PCI Express adapter power cable.
A 6-pin PCI Express power cable requires TWO 12 Volt wires. (Yellow)
Using just one 4-pin Peripheral power cable will cause burning.
Not right away. Takes time.
Everything chilly? I'm 'barking up the wrong tree'?http://www.motherboards.org/imageview.html?i=/images/mobot/motherboards/10420.jpg
Okay, now look at the capacitors on the motherboard.
To the left of the Processor socket are 6 solid Polymer capacitors.
If one of these babies is bad, YOU will know it. They explode.
More of a concern is to look at the other capacitors.
The Electrolytic Capacitors.
More specifically, they are Radial Aluminum Electrolytic Capacitors,
Visual signs of failure for bad Electrolytic Capacitors,http://www.capacitorlab.com/visible-failures/index.htm
LOOK CLOSE. Use a light. A slightly bulged top, or a capacitor on a hard lean, may be hard to see if this is your first time.
Then your Power Supply cr@pped out.
Each PCI Express x16 slot is capable of providing 150 Watts.
Each 6-pin PCI Express power cable is capable of providing 75 Watts.
[ PCI Express;http://en.wikipedia.org/wiki/PCI_Express
Scroll down, read under the bold subheading - Power
A LOT of power is required for the two cards alone, and I do not even know what mobo, processor, ram memory, fans, optical drive/s, and harddrive/s, you are running.
Appreciate it if you would post back with your findings.
Post in a Comment.