There are a couple ways to display your menu but you need to make sure you don't get too confusing either. Most sites I see that offer the two types of menu do so by providing top level only, eg.. no dropdowns, and, on each of the top level tabs, their child pages have the left han menu which is usually without fly outs or sub menus either, and this gives further depth to navigation.
On my site I have this type of menu but find you need to have a good site map - or perhaps good instructions under each of the main menu areas to indicate what the sub levels provide.
To have flyouts on both top and side menus I think could be a recipe for a disaster as unless alot of time has been spent to clarify information, people can get stuck as to where they are within the website.
I used the Links skinobject and the Inventua side menu to create this result - so it's relatively user friendly too.
http://www.xd.com.au and http://www.inventua.com have the Side Menu option as well which can be skinobject and/or module.
If you want to use the standard solpart or DNN menu - simply add display: vertical to the ascx file for it to display vertical instead of horizontal. I have a couple of free skins that have the vertical menu displayed as well.
Hope this gives some clarification.
Nina Meiers