Ok
#1 here is the bios settings I used out of the gate, they have been modified to my taste, but these got me working stable with OC, mind you some of the reccomends are a little old.  They do update the settings but they have been alittle biased against this board.
http://ht.hardwareanalysis.com/content/topic/38623/
#2 plug in that molex, yes it is supposed to be for multiple cards, but if your power supply is not stable or having problems then this may correct the problem.  This plug supplies power to the PCI-e bus.   It seems that you are having your problem at extreme condiotons, I would say that one 6600gt still may not be able to handle ut2004 I have seen several reviews that back that up, if you want I can pst links but basically they are google searches.  Have you changed res on the game at all? One other feature I just remebered is if the threshold of the cpu or gpu gets to hot it will clock back, as you have stated you ran p95, but p95 only tasks the cpu, 3dmark03 does not tax my system as hard as running BFV, farcry really stresses it to max(settings all the way up) and I am running SLI.  Check your threshold settings in asus probe make sure they are high enough.
ESP if you are running  loads other then normal(ie fans lights coolers)
#3 follow the suggestions for the pci-buss, 33.33 and 101 
#4 peg link is the video OC feature as it says in the bios guide disable it. then auto auto on other two settings.
#6 reserved for future use.....
#7 PSU unit should be sufficent, allthough it is recomended to run higher then 450. If you and your friend are running exactly the same setup this could be the problem.
 
#8 as suggested by sxr7171 check your ide driver, and check the troble shooting section under hardware(this is sorta a last guess).  
at this point I would read the the manual cover to cover, this has been my adivce to everone anyway.  A full system spec would be advisable maybe we are missing something, I mean everything. temps would also be helpful including the gpu, current timings etc.
here is one thing I picked up in hardware anaylis I know have switched cards but it might be worth checking into......
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Just wanted to say that I fixed my problem. (Graphics Glitching - System Crashing on 3D intense Stuff) The problem was a faulty card!!! Took back to supplier this morning and got a replacment, tested it there! All works fine now!!
Thanks to everyone who has posted on this forum. 
Just a quick question though. On startup screen (at boot-time) where lists devices, why is the nvidia ethernet not displayed? The marvell yukon shows up when it is turned on but not the Nvidia, even though it shows up and works fine from windows!!! 
Anyone with an explanation for this?
My Spec:
ASUS A8N-SLI Deluxe (1006BIOS)
AMD 64 3200+
Leadtek PX6600GT TDH Extreme Version 128mg (1 only, and works fine now its replaced!!!)
2x 512 Kingston Ram 400Mhz
120 Gig WD IDE
PIONEER DVR-109 D/L Burner
400 Watt MGE LC-B400ATX PSU - 17amp +12 Volt Rail (No Probs with this config)
Logitec Cordless Keyboard + Mouse
Windows XP Pro SP2 + Windows 64Bit
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