Never played FO3 before, but just bought it on Steam today.

List some of the top mods I should install from Fallout 3 NExus.

None yet. Get your game working first. Then start to add mods. When you get your game running smoothly without freezes or crashes, I am sure everybody here would be glad to give you some guidence for mods to add. Please read this topic: http://forums.nexusmods.com/index.php?/topic/652876-fallout-3-modding-and-you/