I've got exactly the same issue as you've Lun4tic. I was running 2 x GTX 465's on an ASUS P7P55D-E Pro (P55 SLI mainboard), and yeah, depending on what drivers I use, On bootup I either get both cards "working properly" but no SLI option, or 800x600 res and get both cards throwing a code 43 in device manager, "Windows has stopped this device because it has reported problems. (Code 43)
This is the device status on both cards. If you disable the cards from Dev mgr, then re-enable them, they both start working, and SLI immediately works, no reboot required.
If the machines boots up successfully starts both cards, then there's no SLI option. Sometimes, if you leave it for a while, the SLI option appears. I've read elsewhere that once you have the SLI option, a workaround is to disable SLI before you shut down, then you have the option to enable it again when you boot up again. This however, would annoy the hell out of me, and I shouldn't need to do it.
So.. here's my specs.
Asus P7P55D-E Pro mainboard,
COre i7 860,
4GB ddr3 1333 (transcend I think, just standard ram)
2 x Identical Asus GTX 465 1GB cards (has voltage tweak on the box, but I think that's just "marketechture" Both cards bought at the same time, only a few numbers between the serials.
Intel X25-m 80gb SSD
WD Green 1TB hdd.
Windows 7 pro x64
Corsair HX 850w PSU.
Originally had Enermax Liberty 620w but upgraded as it was struggling a bit
Drivers I've tried,
Fresh install
258.96 = no SLI option.
260.63beta = Code 43 on both cards. Disable and re-enable both cards, and it works fine.
Also tried 258.96, then 260.63 over the top = no SLI option.
I've run benchmarks (vantage) with both cards individually, ~14k with both cards in SLI (when it's working) ~24k and with medium OC's on cards & cpu ~29.5k.
While benchmarking there has been no instability, no artifacts, nothing to indicate that things aren't working properly.
Initially I had the cards working in the same OS with 258.96 drivers, but had trouble getting physx to go, Had some other issues with the OS so I decided to reinstall.
Reinstalled with both cards in, and in first ran across this issue. Tried a full software rebuild with just one card, Installed drivers, successfully ran vantage, then added the second card. Interestingly when I added the second card it detected as a "standard VGA adapter" and I had to tell it it was in fact a 465. It immediately worked and SLI was available till first boot.
So anyway, I'm still stuck here. Not sure what to try next.
Will keep poking around and see what I can find out.