I used to be an account officer in a bank for these accounts. Net usually denotes the individual's full proprietorship of the net worth and not in association with others (unless from family) and should not have the literal translation (i.e., صافي) and thus confuse the meaning. Here is a reference link which would not be accepted in the space allotted below:
http://translate.google.com/translate?hl=en&langpair=en|ar&u=http://www.investopedia.com/terms/h/hnwi.asp&prev=/translate...

Here is the original English:
http://www.investopedia.com/terms/h/hnwi.asp

--------------------------------------------------
Note added at 10 hrs (2009-03-22 21:11:22 GMT)
--------------------------------------------------

You could also say:
سخص ثري أو من الأثرياء