I have used Maya in a production environment for several years as an artist, across multiple versions, and I have to say that learning a newer version of the program is going to be more beneficial than sticking to 6.5.
If you are truly serious about getting back into the program I would recommend that you shop around and find an updated textbook- the investment will pay off!
Although the fundamentals of the program have been stable since version 6.5, there have been many improvements to the user interface, the scripting editor, the render settings, better view-port previews and much more. Being aware of these differences is important.
Make sure to check out the 'Whats new in version X' website, for example this website for Maya 2011. Checking out these pages will assist you in making an informed decision about the software package you decide to learn.
In addition to the new improvements, bear in mind that many new plugins that you may wish to use with your work may not support older versions of Maya, and any plugins that do may no longer be supported by the people who wrote them.
Finally, assuming you want to use Maya in the workplace, learning an outdated package may mean that your skills won't be as appealing to a prospective employer, unless their pipeline is also built around older versions.