For low light, I think the D3X is out of the question... the highest "normal" ISO rating is 1600... while you can go higher the Hi-1 or Hi-2 (3200 and 6400) you are going to see a LOT of noise, especially in low light.Those levels are not true ISO settings but "internal camera manipulations" to acheive those ratings.
The D800E, while having a 36+ mp sensor, which sounds great, also has its problems... even if you use a tripod for your shots in low light, if the wildlife moves and your shutter speed is not fast enough to catch it... such ultra high resolution will cause a lot of blur... the 800E doesn't have the low pass filter and as a result has increased problems with the possibility that moiré and false color will appear.File size will chew up memory cards fast, you will need an external hard drive to store the files, etc.(See the Nikon site for the pros and cons of the D800/800E)
I have the D3S and have shot in light that was extremely low(perhaps not as low as you may want to shoot in) and obtained very good results.
I think you may be better off with the D3S... higher ISO with very low noise, 9 frames per second ... a really true classic