You get to keep Logic 9. They have different names in the app folder, so no problem.
From what I can see (and I've downloaded all the additional content), it looks like all the Logic Pro X stuff went into the main Library/Application Support/Logic folder, whereas all my Logic 9 stuff is in the user/Library.....
Anything that's 64bit will work. (so no Sylenth among others) I think everything NI is 64bit.
There's soooooo much new stuff, tons big and small. It's like FCPX almost, once you get used to the new stuff you won't want to go back. One of the many things is that now you can export audio to Quicktime no problem (it's only 64bit now) among the many other things. Great writeup here: