One problem with target="_blank" -- if you have multiple links that you want to pop up, you start bombarding the user with new windows. To use the same window, give it an actual name ... like target="theWindow" or something.
One problem with THAT is that if "theWindow" already exists, the next time a link is opened in it, "theWindow" will not autmatically come to the front so that it's visible on top of other open windows.
Because of that I prefer the javascript window.open() function, because it solves the problems above, and it gives you control over the features, size, and location of the window (which toolbars to include, can it be resized, etc).
<a href="javascript:w=window.open('_url_');w.focus();">linktext</a>
(I'd usually put all that in a function.) You can read up on the window.open function somewhere...