I know how to OC a video card but if you have 2 video cards how do you do it?

Using Rivatuner doesn't work? I have never done SLI/X-Fire so I wouldn't know.
 
I'm sure one way that'll work flawlessly is to OC each card separately. Then flash each vidcard's bios with the slowest of each setting (Core, Shader, Memory).

IIRC when SLi'ed, the cards will OC simultaneously to stay in sync.  I could be wrong, havent personaly tryed it
 

Generally the cards will clock to the primary card.  This is why it is recomended to put your slower card in the primary slot (if you bought from different manufactures).  I don't know if you can OC simultaneously, but I would like to know.  This could save me a lot of time in the next few weeks as I OC my 9600 GT's, but that's not as much of an issue now that school is out.

from what i have read the slower will sycn the clock speed to the higher speed card if its in the top(primary) PCI-E slot.
 
but make sure your card can run stable at those speed.by testing it first just with one card.and OC one card to your desire clock speed then plug the other one in they both will sync.
 
if you really want flash BIOS on both card,one by one is the safer way.but i heard you can do it when two card is plugged in.
 
again i have had any experience on it.all just based on reading.

 
 
You need to first test the OC settings for both cards, one will be faster than the other, whichever card is the slowest use it for your reference to flash the settings to the video card Bios, don't use the maxxed settings of the reference card back it down a little to give yourself a safety margin.
 
Then bios flash the core/shader/mem settings of the slower card and test it, don't flash the second card until you're sure the first is rock stable, don't flash the first card with the second in the machine, because the flashing program will flash them both one at a time, and once the process starts you best not stop it so only flash one at the time while you're still in your testing phase, if you're absolutely positive your settings are good you can flash them both in the machine, but be sure first!
 
If you screw up the first card you'll have the second to get you back to the flash point to correct the problem, study how to flash the card and familiarize yourself with the procedure thoroughly, www.mvktech.net, is your study and download source for the guides and programs you'll need like NvFlash and NiBiTor, IF!!! You have any doubt about how to do it, DON'T!
 
Cooling is very majorly important when OCing an SLI setup, the 2 cards tend to heat each other, so I would reccommend after market cooling, but if you go stock cooling check its contact with the GPU die, don't assume anything, and add increased fan speed into the bios flash.
 
Remember to allow some leeway because an individual card may OC higher than it will when the 2 are side by side stealing each others airflow, once both cards are identically flashed they'll run in SLI just the same as if they'd come from the factory like that.
 
Definitely Been There Done That Successfully!
 
However you do this at your own risk! Good Luck! Ryan

Or just use rivatuner instead of bios flashing voiding warrenty risk braking it etc etc???

So Rivatuner does OC SLIed configs.

Yes, Rivatuner works for pretty much any setup including Quad-SLI.

umm i just overclocked my ASUS 320mb 8800GTS's core and memory clock via nvidia control panel. Is that bad?
 
I used it to overclock from 513->630mhz(core) and 792->900mhz(memory)
 
@ stock they idle at 53 and 62 degrees Celsius  
@ stock under load 70 and 76
 
@ overclock idle is 53 and 62
@ overclock under load 74 and 79
 
people say these cores are good up to 100 degrees so i could possibly go further and i'm still on stock cooling!

Well, 100C isn't exactly a good thing to do. Generally, you want to stay below 85C, and besides, without a volt-mod, there's no way you can go higher than, say, 700MHz for core.