When I bought my current computer, it was loaded with Vista and had an onboard vga. I installed a GeForce7600 rather quickly, and had no issues with installation.

8 or 9 months ago I upgraded to 7 Ultimate, and still had no issues with the 7600 CARD. But, in the last month or so, I've had many crashdumps and general display issues related to the card, and I figured it was time to replace it.

So I went out and bought a GeForce8400 on the cheap (college student budget, ya know). The instructions that came with it specified that I should uninstall the drivers for and disable the onboard vga - well, I couldn't, because the onboard has never shown in the device manager since the day I upgraded to 7. So, I followed the rest of the instructions.. and.. the first time I try to boot up the PC, there's at least 10 seconds of listening to something inside the tower make a sound kind of like a mix between a "CLICK" and a "pop"... it's like.. "click/pop".. "whiirrrr".. "click/pop"..."whiirrrrr"... etc, for 10-15 seconds, and then it boots as normal. EXCEPT, the new card is completely ignored and the only way I can get a display is to plug into the onboard.

I've installed the drivers that came on the CD that was included with the card. I've tried updating them as well. But the card doesn't even show in the device manager. I've changed the BIOS to PCI-E, and PCI, but neither one worked. I've checked the fan on the board to make sure there was nothing blocking the blades but it looks completely clean and clear to me.

I'm at a loss for what to try next - I'm not much good in the hardware department. D:

Oh.. basic PC info:
Compaq Presario PC (SR5510F)
AMD X2 5000
Windows 7 Ultimate 32x
2GB RAM
Onboard is an NVIDIA GeForce 6150 SESounds to me like the hard drive is getting ready to fail....i never heard a vid card do a clik/pop.......whirrr...
Remove the new card so at least the machine functions fairly well...follow this Slave Drive TUTORIAL  to get your important data...use the machine as little as possible and shop for a new HDD.I've already removed the card, mainly because if I can't figure it out soon, it'll have to be RETURNED. The click/pop/whirr doesn't happen anymore once the vidcard is out of the picture, but, now I'm stuck running on only one monitor with so-so display due to the blah onboard vga. D:

Without the vidcard in, I can listen to the HDD and it's almost inaudible.. I have had no issues with it at all. I'm kind of hard on my PC's so I'm (UNFORTUNATELY) familiar with the sound of a dying HDD, and this one definitely does not fall into that category. The click/whirr thing had to be related to the vidcard, since it started on install and stopped on uninstall.

Is it possible I don't have enough power to run this card? I really am clueless as to how I can check to see how much power my PCI slot has access to... and this card requires 300W.OK.
I sit corrected on the drive issue because basically i'm not there to see/hear what's going on...but as stated i've never experienced that from a vid card...
That being said return the card for a replacement...meantime find out what make and Wattage the PSU is and we'll go from there...
Did the vid card have a 4/6 prong plug on it that may not have been connected ? ?Well, if I had a way to do it, I'd record the sound and share it with you, lol. 

Is there an easy way to find out the wattage? Or even a less easy way? I really just don't know where to look. I'll probably go and exchange the card in the morning and see if the replacement has anymore luck.

I didn't see any prongs at all.. I don't think it has to be connected to any additional power source. I doubt it is a power issue as the 8400GS only uses a minuscule amount. It also does not require auxiliary power.

There was an incompatibility with some models in the 8*** series (which used PCIe 2.0) when used with older PCIe 1.0a, but I can't tell if this applies in your case.

It seriously sounds as though it is simply a dead card. Go ahead and replace the card.