NFS file consistency

Hello All,
    I am trying to track down some file consistency problems I am seeing
and I want to make sure my assumptions about NFS are correct.
    Say I have 2 NFS clients, machine A and machine B.  Machine A does
an open/write/close on a file.  After this machine B does an open/read on
the file.  Is machine B guaranteed to read the same data that A wrote
or is there a delay between the time A closes the file and the time B
can expect to see valid data?  Also if the file already existed before
A wrote it, and B had already read from it and closed it, does this
affect anything?

Thanks,

Jim