Sebastian, thank you for your comments and advices!
In last days I upgrade my site from 5.2.3->5.4.4->5.6.3
(I use your comments in this post: http://www.dnnsoftware.com/forums/threadid/435849/scope/posts/threadpage/2/what-upgrade-path-can-i-use-to-go-from-dnn-49-to-dnn-6 )
After I make this step 5.6.3->5.6.8->6.2.8. All stage was without errors.
It looks, that 5.6.8 last build, that works correctly with IE8.
Site on DNN6.2.8 looks in IE8 very strange. Is it possible to correct this issue?
I thinking about upgrade to last 7.x build.
Can you give me advice obout next upgrade .Net and SQL and IIS? Now my platform is Server2008R2, IIS7, .Net3.5, Sql Server2005sp3.