If one converts and not the other, the marriage is still valid for husband but for wife its a grace period before she leaves him unless he too converts.
If both convert then their marriage is also still valid.
If husband converts and wants to divorce he's wife than he can do so. It would good idea to preach Islam to her first to see if she would convert as well, if not than he can leave her and take a muslim wife for the sake of his kids and their future.
I"m guessing in terms of atheist or pagans, the marriage will have to end unless the partner converts (husband or wife). Whether there is a grace period or not, i'm not sure but i would guess so considering its the a similar situation to the above.
Anyways, read this...
http://qa.sunnipath.com/issue_view.a...ID=176&CATE=11