I've run Apple Logic Pro 9 on what is most of the time used to run Pro Tools HD, but is occasionally used for Logic and other sequencers. I can't say that I've ever had an issue running Logic within this configuration, no matter how many tracks I'm using at a time. Keep in mind that of course since this is built for an HD system, it is a very good computer and system in general that is powerful enough to handle just about anything. For those of you running Apple Logic Pro 9 on lesser system, you might encounter some issues when you start having a lot of tracks and effects at once. It really just depends on your system...
From right when I started playing with Apple Logic Pro 9, I could tell that this was a big step up from Logic Pro 8. I thought Logic 8 was put together very well, which it is, but it is definitely worth the upgrade to 9. It has a ton of new features to provide more control, as well as new virtual instruments and sounds. It doesn't cost all too much to upgrade, so I'd highly recommend doing so if you have a previous version of Logic. If you don't and are looking for the most complete software for composing with MIDI, look no further than Apple Logic Pro 9.