US citizens must file a 1040 declaring all of their world wide income, no matter where it came from. It doesn't matter if the money is earned in US dollars, Singapore dollars, or Zimbabwe dollars... you select a conversion rate for the year.
However, all Americans are entitled to an earned income exclusion for income earned overseas. For 2013 it was $97,600, for 2014 it is $99,200... thus although your wife will need to file, all of her income will be excluded from tax. You will need to file form 2555 in order to claim the exclusion.
Note that you must meet the qualifications for claiming a foreign country as a tax home, which in simple terms means that you have resided in the country at least 330 days. The details are more arcane.
Note however, that all unearned income (interest, dividends, etc), as well as your US based salary will still be taxed at a rate as though the earned income exclusion wasn't in place.
If she receives housing or certain other benefits, she may also be able to qualify for a housing exclusion allowance. Generally, this occurs when the company is picking up some or all of the rent.
You can read all about it here: http://www.irs.gov/Individuals/Internat ... -Exclusion
And of course, she must file taxes as well with IRAS in Singapore, a far simpler process.