Database Software

I have the following questions in working with the Simplified Chinese characters in Oracle:
The Simplified Chinese characters can be loaded to the NVARCHAR2 column of a table using SQLLDR with characterset ZHS16GBK defined in the control file and can be displayed in SQLPlus. They can also be inserted through linked table of Microsoft Access. But why the Simplified Chinese characters cannot be inserted directly through SQLPlus?
Why the query cannot be executed properly (i.e. no record is selected) in SQLPlus or through linked table of Microsoft Access, if the selection criteria or the WHERE clause contains the Simplified Chinese characters?
Do I need to create a unicode database (i.e. database characterset = AL32UTF8 and national characterset = AL16UTF16) in order to solve the above problems?


For you information, the current configuration / settings of my working environment is shown below:
Server side
Operating System: AIX 5.3
Oracle Database version: Oracle Database 10g Enterprise Edition Release 10.2.0.3.0
Oracle Database characterset: US7ASCII
Oracle National characterset: AL16UTF16

Client side
Operating System: English Microsoft Windows XP Professional Version 2002 (SP2)
Oracle Client version: Oracle 10g
Locale: Chinese (PRC)
NLS_LANG in registry: AMERICAN_AMERICA.ZHS16GBK
Microsoft Access version: Microsoft Office Access 2003 (11.6566.8122) SP2

Many thanks in advance.