well, its really hard to tell you a specific value for the sun and ambient value. I'd recomment you play around til YOU think it looks good, but make sure you write a function where you can change those values in realtime while you run around the level, you can also find a similar code in the Aums or even in the Manual i think.

Night Skyboxes you can find all around the Internet or even the forum or Free Recoures Site.

Fog makes the Night Scene even more atmospheric, you can check out Helghast Fog Code for that (Search for it).
Also make use of distance fog (manual).

OR...

just take a look at MystyMood, which has dynamic day/night change.