Get the info about the the processor you are looking at.
In the past there was the Pentium 2 and 3 Xeon and then just Xeon when the P4 came out.
Some retailers differentiate between socket 603 and socket 604 by calling one P4 Xeon and the other Xeon.
However the only way the find out what you are buying is to look up the details.
Have a look
here.
Hope this helps