Comp not recognizing video card

Hi,
I have two video cards 7300 gt and an 8600 gts. My 7300 gt card used to run fine in my gateway PC (AMD X2 4200+, Foxconn C51GU01 motherboard,500 W power supply with 2 12V rails at 18Amp and 16 Amp, 1 PCIE x16 slot, XP Media Center 32 bit) but after I over clocked it using n tune last month it ran fine for a couple of days and then it caused the screen to flicker (horizontal lines) and eventually freeze/hang the system at various time intervals after I booted into the system. Then one day it eventually stopped recognizing the card. I RMA'ed the card thinking that there was some fault in the card and got a replacement for 7300 gt. Also I bought a new 8600 gts card. Before I put the new cards I did a clean install of XP and also updated the BIOS. Also I disabled the onboard n6100 series graphic adapter in the safe mode after uninstalling all nvidia display adapter for it and using a driver sweeper for making sure I removed those.
1. The 7300 gt card was recognized by the system but again started freezing the screen. After this happened I was advised to update AMD processor driver and AMD dual core optimizer and also get NVIDIA beta drivers but they did not help. After that I just went to BIOS and loaded failsafe defaults and then viola it ran for 2 days without a problem but after that it crashed and is not recognized by the system.
2. The 8600 gts card is not even recognized by the system. I can see and hear the fan running when I boot up the system but when I go to device manager it does not even acknowledge the card being there. I also tried to add new hardware but that did not help either. When I did contact EVGA they told me that I might have to reflash my BIOS as older BIOS dont have support for 8000 series car. Now I had already updated with the latest available BIOS from gateway as Foxcon does not support it.

Please let me know what else I can do. As this is a micro BTX mother board only gateway keeps a replacement but it is too expensive. I could just buy a socket 939 ATX motherboard and an ATX case and use existing parts which will be a cheaper alternative. But I want to do this only after I make sure it is realy the hardware causing problems not the BIOS.
Thanks.

 

Pm Solaris17 about this.. He is your 8xxx guru and should have the ability to help you on it... I' don't know much about nVidia cards so I cant help, but I just know of the person to talk to for the questions..
Good Luck and Welcome To TPU