I am totally going from memory here, but if my memory serves me well (someone correct me if I am wrong):
Windows Vista or + or Server 2003 or + (I have 5.x.x instances working on Windows 10 and Server 2012 too)
Asp.Net 3.5 (but also works on 4 and 4.5)
Sql Server 2005 (also works on 2008 and 2012)
Basically, except for Microsoft Azure, it should run on pretty much anything.
That being said, I would at least upgrade to 5.6.8 which is the latest 5.x.x release with bugs and security fixes, etc.
Then and only then I would upgrade to 6.2.8, there are no breaking changes, what changed from 5 to 6 was mainly a switch from VB to C# + bug fixes, optimizations, better UI easier to manage sites, etc.
At this point I would analyse which modules are in use and have updates available. Dnn 7 had some breaking changes regarding the services framework, most open source modules have been updated for Dnn 7 support but there are a few exceptions. If you have paid modules, then you may have to pay for newer versions. If you decide to go ahead, then I would upgrade to 7.4.2 which is the latest in the 7 series.
If everything works fine, then you can safely update to 8, the main difference is added support for MVC and SPA, but everything that works on 7 will work on 8. I would upgrade to 8.0.4 which is the lastes 8 series release.
Now you are one step away from the latest version and you need to decide if it is correct for you. I love the way the UI is made, it is much faster to manage sites in my oppinion.
But, I haven't used 9 in productions for a couple of reasons, first I can't manage to compile from source correctly so it prevents me to make quick patches if I need to. It lacks the option of having page icons for menus that use images such as a MegaMenu. And a few more small things, Chris Hammond outlines a couple things at
http://www.chrishammond.com/Blog/item...You can find the older versions at:
http://dotnetnuke.codeplex.com/releas...And the newer versions at:
https://github.com/dnnsoftware/Dnn.Pl...I have upgrade many sites all the way from version 3 and there are a couple more old timers around, so feel free to reply if you encounter any issues.