Before going any further, please click on "Common Game Issues" in my signature, and run through all the steps there. Post back with results.
In one of the NVIDIA Forum threads you linked to, you said you were using the 169.04 drivers. Those are Beta drivers, and can't be considered stable; go here
and download the latest release version drivers - 163.75.
I do not think the power supply is the problem, although I would be running that system on a 1000W. Your 850W should be able to handle it though.
I did say that I used the 163.75 drivers in the links I posted in my OP, but I still get the same issues. That's why I went to 169.04, but instead of nvidia's site I went to XFX's site and am now using the Official XFX 169.04 drivers as of now.
I already read the stickied post about what needs to be done before posting on the forum, I can assure you my system meets the specs, I can run the game with the CD or through a backup and it still occurs. In SLi it's worse, in Single card mode it still occurs but not as bad. I can assure that maybe NFS Carbon is not rigged up to handle SLi, but the problem is not entirely resolved with just the single card. The game is patched to v1.4 and I restart my computer every time I do anything drastic like change drivers, install something or get a nvlddmkm.sys error.
**note about the nvlddmkm.sys error** It seems I get this only in SLi mode, but I'm testing it thoroughly. I thought I narrowed it down to the issue occuring with the Vista GUI and the Nvidia drivers or DX10, but that ruled that problem out when I got the error on Windows Classic GUI with no addons running such as WindowBlinds that would attempt to force some features of the Aero GUI to run. So now I'm just running 1 card with the SLi feature disabled; like I mentioned above, NFS Carbon runs a bit better, but the drop to 0 FPS for 1-1.5secs still occurs but not as frequently as it did before, and it doesn't stall out as much on some maps as it does on the first map I tried. Where my example was 3-4 stalls during a race of 2 laps around the track, where on a mountain drift run it might stall out 1 or 2 times.
It's hard to believe though how smooth the game runs for it to abruptly stall out like that.
Here's my dxdiag just in case anything else may be needed from it.
--- Also let me give you a listing of games I play that DO play fine.
WarCraft 3 - FT (Runs with SLi fine, even though there's no benefit)
Unreal Tournament 3 Demo (Ran SLi Fine)
Titan Quest (Ran SLi Fine)
Supreme Commander and FA (Runs fine in SLi, never seemed to have an issue with nvlddmkm.sys error with this game)
Oblivion (Runs in SLi fine, until nvlddmkm.sys error shows up)
RF Online (Runs in SLi fine)
Need for Speed Pro Street (Only runs good with 1 card, SLi not supported)
Need For Speed Carbon (Already a given)
Lineage 2 (Runs fine in SLi)
Hellgate London (Runs great in SLi, except for the nvlddmkm.sys error)
Halo 2 (Runs fine in SLi)
Gears of War (Runs fine in SLi, havent had an issue with it yet)
FarCry (Ran fine in SLi when I played it, had some BSoD errors but that's when I first got my 8800GTX's and drivers weren't mature, ran good after that)
F.E.A.R (Runs fine in SLi)
EverQuest 2 (Could run in SLi but FPS dropped, only supports single card)
Company of Heroes - OF (Ran fine in SLi, until nvlddmkm.sys error occurred)
Call of Duty 4 Demo (Ran fine in SLi, no issues)
9 Dragons (Ran fine in SLi)
Half-Life 2 (Ran fine in SLi)
CounterStrike Source (Ran fine in SLi)
now most of these games ran good, the nvlddmkm.sys error is very random it could happen 5mins to 2hrs to 2 days later from showing it's face. It's a very odd error and something new to me with Nvidia cards, honestly I've used ATI up until Dec last year when the 8800's were out. I never once had an issue that would disrupt my system with ATI like I am now with Nvidia, ultimately it's greatly irritating.
So the games I did not receive that error on doesn't mean that the game itself was immune to that error, it just probably wasn't that right time, whatever in the hell that means I dunno, but that's just how the error seems to work, it want's to show up when it feels like to.
I could play maybe 3 or 4 different games after another without the error, really it hasn't been that big of a deal for me and really I remember having this issue before a long while back and haven't seen it since so I really don't know what was done to fix it, if it even was a driver issue or not. Some people recall not even seeing the error at all so that's just some added information to include with my main problem NFS Carbon's performance drops.
I don't experience any BSoD's, no crashes, no freezes or random reboots, nothing out of the oridnary. My BIOS are at F7 and my drivers on all my hardware is at the latest version or firmware.