I used to run an ATI 2900XT and could get my E6600 to run stable at 3.4Ghz. I upgraded to an Nvidia 8800GT and ever since have to run my CPU at stock clock. I've tried increasing voltage on the CPU and PCI-E. I've tried increasing the PCI-E bus speed. I can't think of anything else to try. I know its not the CPU because I put my old ATI card in to see if I could still hit that speed and it did no problem. When I inserted the 8800GT I couldn't get past POST. I use a Gigabyte GA-965P-DQ6 motherboard. I'm new to Nvidia so I wasn't sure if this is a common problem for their video cards or maybe I just got an unlucky card. Any idea's or suggestions would be greatly appreciated. Thanks.

Full system spec - including PSU?

Yeah it sounds like the PSU might be the culprit.

+1 PSU... although heat could be a small contributing factor too it's not your main problem.

-mcg

Did you completely remove the old ATI drivers before putting in the 8800GT and installing the drivers for it?

Power requirements:

A minimum recommended 400W system power supply (with 12V current rating of 26A). A +12V 4-pin supplementary power connector

Didnt the 2900xt drink more than a scottish priest? I woulda thought that the 8800gt would consume less power??

lameness is right. The power requirements for the 2900 XT are:

Connection to 550 Watt (750 for CrossFire) or greater power supply with two 2x3-pin PCIe power connectors is required.

It could be either the video card or the power supply. I would start by RMAing the video card.

The PSU is an Antec 650W. I don't think power is the problem. I really don't want to RMA the card because it works. I just can't OC the CPU when using the card. I can live with it for now just wondering if anyone had heard of this problem before? Didn't know certain components could lead to poor overclocking results. I completely removed the old Catalyst drivers before installing the new card. Thanks for the responses everyone.

how are you overclocking the gpu? Are you using rivatuner? You mention you increase the pcie bus speed, and if you increase that, it can cause errors and data loss.

There is not much difference in performance between a 2900 XT and an 8800 GT.
Did you use Driver cleaner to remove the old drivers?
A while back there was a problem with the 8800GT and power supply.
A lot of cards got RMA'd and power supplies changed.

I use RivaTuner to clock the GPU but I was pushing the PCI-E voltage and the bus speed a small amount in BIOS. I figured more power and speed to the bus might make it stable but no luck.

There is a huge difference in the 2900XT and 8800GT in the games I play. (LOTRO and Crysis mainly) I used driver cleaner.net to remove the ATI drivers. I have one of the early 8800GT's meaning I bought it overpriced before they were in steady supply.