I recently got a core i7 920, 3x2gb corsair ram, and an evga sli 758.
 
I installed windows xp 32 bit, and it works fine with one graphics card. as soon as I add a second, nvidia wont see my cards, and device manager shows an error with pci express root port 3.
 
I tested both slots, both work. A work around was to set my dedicated PCI Express memory to 2gb. anything 2gb or higher will let me boot into windows.
 
I tried a clean install of xp 32 bit 3 times, and a clean install of xp 64 bit twice.  Both give the same hardware conflict. xp 64bit even makes me boost this number up to 3gb before I can go sli.
 
This is pretty annoying. I updated my bios, tried to disable everyyyything in bios I dont use to free up IRQ or whatever. No luck.
 
Does anyone else have this issue? or do I need a new board?

Did you try Windows 7? 

Full system specs.
 
i7 920
EVGA x58 sli 758
250gb sata, fresh windows xp 32 bit installed, and tried xp 64 bit. I have an additional 250gb sata with windows xp32 bit, and same exact issue.
2 8800gt zalman edition graphics cards.
my psu is 850watt thermaltake with [link=mailto:69A@12v]69A@12v[/link] rail.
 
Ive done tech support in the past, im sure the drivers are correct, clean installed, and up to date.
 
First thing I did was install my motherboard drivers, then the most recent nvidia drivers. It fails to work in SLI.   I also tried the same with windows 64bit. Same issue.
I am using an SLI bridge, HOWEVER, it is a leftover bridge from a socket 939 DLI Lanparty motherboard. Is this my problem? I read online, and people say an SLI bridge... is an SLI bridge.. 
Both these cards work, I tested them each, and I tested my PCI express slots each. Everything works fine, except when I run them both at once.
 
Even when I do get them to run at once, when I set the bios to 2gb dedicated memory, my graphics are still choppy, even when getting 100fps+
Both these cards worked in my old machine perfect. So I dont think its my power supply, cards, or drivers.
 
My next step is to try windows 7 64bit, if that doesnt work, I guess another SLI bridge. and if that doesnt work.. I might get a new board or go single graphics card and sell these 2 off. SLI is not worth the hassle, and nobody seems to have heard of this issue.
 
And like I said in my previous post, they DO work in sli, i just have to configure my dedicated ram or I get hardware conflicts under the PCI Express root port 3.  and even port 7 on windows 64bit.
 
  PS. I also have an X-Fi gamer sound card. I was sure to disable onboard. But im going to reformat xp 32 bit, and try with onboard sound and no ethernet drivers. See if I still get the same conflict in device manager when SLI.

Can you try with Win 7 x64 and see if you have the same problem? also SLI still works without he bridge, even if you have the message that is not connected.