Good day ! I use Google translator so sorry 4 that .
I have and I realized not only have a problemwith the regime of SLI, which is that there is no difference in FPS between SLI mode and without it. Perhaps it is a problem in the driver nvidia ?Using the latest drivers 290.53beta.
I will appreciateif someone can help in this matter!
Game settings of 1920x1200, Ultra

Do your video cards have over 1GB of memory each? If they don't then running at that resolution and ultra shadows/AA you will use more vram then your cards have and this will kill performance regardless of how strong your cards are. I can assure you that SLI does in fact benefit with Skyrim and that 290.53 driver, just don't do things like set everything ultra when the cards can't handle it.

I have two gtx 280, each 1 GB GDDR3 . I'm using MSI Afterburner on the monitoring values do not ??exceeds the 650-700MB video memory

Yup, don't do all ultra, 4xAA and say 'high' shadows, see how that goes for you. (my last cards were 3xGTX280s and I'd used them with Skyrim so I know what's up)

as video memory should be enough(650 MB),and do not want to drop shadow settings
for comparison,Crysis 2 on average 50-60 FPS with V-Sync, 1920x1200, Ultra and have no problems.

This isn't the Crysis 2 game engine, Skyrim's is much hungrier for the stuff for sure. When you check Skyrim is only using 650mb of vram? Neat, well I suppose yah I have been using 3rd party mods that up the wants pretty much since they started showing up. Either way SLI does help a fair bit. Btw if you aren't using Skybooster definitely get that app and use it with Skyrim, its an incredible benefit. By the way something else I personally noticed with the 290.53 driver and when I was using 280s was at some point while playing the game the driver would shift the cards to only performance stage 1 and wouldn't go back to full performance, you might be experiencing the same. I fixed that by editing the skyrim nvidia profile to pick 'full performance' instead of 'adaptive' and that fixed things.

Just went to a couple of minutes in the vast Skyrim both cards use the same value equal to 654Mb, I think it may change over time, it will be necessary to check more detailed.
Skybooster definitely try, thank you for this!
Google translator-translator svcks, barely understood what you wrote, and the had to read the original text,and not a translation!))
such a problem with power video card was not mine, but I can try to full performance mode

selected portion of text yet I did not understand correctly

Okay let me try that again then. Sometimes when playing the game the 290.53 nvidia driver slowed down the video cards' clock speeds to 'light performance' instead of 'full performance' and it would not change them back. I edited the Nvidia Control Panel's Skyrim Profile's "Power Management Mode" from 'Adaptive' to 'Maximum performance' fixed that. This is something to watch out for since you are using the same type of video cards as I was and the same driver, it would slow the frame rate down to half speed when that happened and I would have to reboot the computer to fix it.

Yeah got it! I had no such problem. But I think that there is no sense in that would change the mode with adaptive to maximize performance, by the fact that video cards use the right power supply to them is controlled by the driver, but could be wrong.

For sure usually that setting takes care of itself but I just happened to run into a problem where it would randomly lock itself at low performance when combined with GTX280s in SLI, more of a warning in case you meet the same situation that's all. Happy Skyrim playin'!

Of course thank you for your wishes and your experience, but I did not see an answer to my question:?Why does not increase the number of frames per second in SLI? ?