As MrDJjim sad two first things that also felt to my mind are also: 1). maybe one 8800GT is dying and now it's showing artifacts under stress caused by demanding graphic engine in Crysis 2 ...2). It's possible that one or both cards are overheating because their heatsinks are full of dust ...but there could be holy bunch of problems causing problems you have right now.
Before turning off SLI check and clean heatsinks on both cards and then in SLI mode try to maximum stress both cards in SLI with Furmark: http://www.geeks3d.com/20120911/furmark ... md-radeon/
..and if you can see artifacts after 15, 20 minutes of stress-test then try to stress only first 8800GT for 15, 20 minutes (pull other card out if you don't know how to turn off SLI in NVidia driver settings), and then second 8800GT for 15, 20 minutes to see which card is causing artifacts.
Maybe underclocking GPU/shaders/memory with MSI Afterburner: http://event.msi.com/vga/afterburner/download.htm
...could help in that case, but leave that for the end after you exhaust all possibilities mentioned below.
Also try to open computer case to maximize airflow in in: maybe 8800GT cards are overheating because lack of fresh/cold air in computer case. If you cant's see any artifacts or experience crashes with computer case fully opened then add few fans into compjuter case to take the heated air out from computer case more quickly.
Maybe PSU is overheating or lacks of power on +12V rail/rails when SLI is working. What PSU are you using and how much curent it has on +12V rail multi +12V rails: +12V1 = ?; +12V2 = ?, +12V3= ?... If you can try with stronger PSU.
Also, if you already didn't do that try to change display drivers to 306.97 WHQL: http://www.nvidia.com/object/win8-win7- ... river.html
Try all this above, one by one and then post here if there is any change... I have few other things in mind but this is enough for start...