SLI freezes computer when enabled

I bought a bunch of components about 3 weeks ago, but I have had to replace the motherboard (I was sent a a discontinued model that didn't support the RAM timings). I just put in a EVGA P55 Micro (just found out that it is EOL), but everything now works - except SLI.
Computer Specs:
i7 870 at stock speed
4GB DDR3 1600 G.Skill Ripjaw
EVGA P55 Micro socket 1156
Win7 64bit
2x MSI GTX 460 1GB Cyclone
Seasonic X Series 750W Gold

Let me backtrack a bit...
I had a P55V, but it wouldn't run the RAM at the rated timings, but it would run SLI. The place I bought it from could not guarantee that I would receive the correct version so I had to buy a completely different model. Just the other day, I installed a P55 Micro and it finally allowed me to run the correct timings. But not SLI...

Here's the problem:
Whenever I try to use the Nvidia control panel to "Maximize 3D performance," the computer will show the loading bar, the screen will flicker while it begins to use both GPUs, and then freeze. Sometimes I can move the mouse for a second or two before it freezes, but none the less freezes.
I thought it may have been a video card malfunction, so I switched their placement and tried it again. Then I tried switching the SLI bridge around. still nothing.
Uninstalling and reinstalling new drivers does not help either

Games run fine on one GPU, but given that I was able to run SLI before, I'm wondering if it is a faulty electrical slot on the motherboard...

Ideas?

It might be the motherboard. Try swapping the GPU placements (and only running one card at a time) to see if anything weird happens. The one card at a time would be the next time instead of just swapping and trying.

You mentioned some RAM stability problems as well... Can you run a memtest on that, just in case?

The RAM was related to the P55V - it downclocked the 1600 to 1333. The P55 Micro runs 1600+. I'll test each card in a few moments.

I wasn't having stability problems with the RAM, the P55V only ran 1333 and would not allow me to run at 1600 with the rated timings (it was a discontinued model repackaged as the new revision).

Anyway, I can't test it out anyway -no dvd drive.

Both video cards work separately. I did uninstall all the audio drivers (read it on the EVGA forum about x58), but still wouldn't run in SLI - I could enable it, but only the first GPU was ever used.

EDIT:

I installed both cards and placed the SLI bridge on. Fired up and it froze on the Welcome Screen...twice, currently in Safe Mode.

Finally got it to work.
I had to uninstall all the drivers in Safe Mode, then install the EVGA ethernet driver and Nvidia drivers. Flashed the lastest bios. Then had to reboot several times. Finally had to disable Nvidia HD audio in the appropriate bus -es to ensure that there wasn't a conflict. Finally got it to work and made sure checking I had everything correct. Ugh, the bios update was the worst, I still need to set my RAM timings...

Glad you got it working.

It simply amazes me as to how some problems occur and how they are fixed... Computers should be that problematic.