I have version 3.0, Producer. Never upgraded because I never had the need to - with all these VST issues I am having, however, I will probably be upgrading soon. For now, I get by with the DXi.
I forgot that the Sonitus fx do not come with it. My friend works for Cakewalk, and he lets me try out a whole bunch of cool Sonar plug-ins The sonitus ones are amazing!
About the memory warnings, you should expect those, but I believe that with everything loaded, it is just under 512mb. What I had to do when writing was load most of the instruments (excluding the big ones, like the harp!), edit, unload some, load others, edit, etc. When it was all sequenced and fx\'d up, I loaded all and exported the audio. Getting the extra 512mb has been sooooooo worth it! Can\'t imagine going back to just 512 hehe!