I just asked a Chinese friend (who lives now and always has lived in China, and the same one who informed me Indians were called 阿三); here's what she said:
指美国人？？？ [referring to Americans???]
美帝算么。。。 [does 美帝 "American imperialist(s)" count...]
指白种人也行 [or referring to white people]
有吗 :p [is there such a word :p ]
洋鬼子？ [ghost?] [ok, I've editorialized that to match your wife. But I don't see the problem with glossing it that way; 洋 explicitly refers to foreignness, but 鬼 really does mean ghost.]
你怎么问我呢？ [why are you asking me?]
呃。。那应该就这一个了 [oh.. then it should be this one]
The worst the Japanese seem capable of is "white pig" (specifically towards Americans). I mean, it's more accurate than offensive...
Tone (and context) matters more than the word though. Calling someone silly in the right tone could come off as offensive.