Quick points here:
– Your 2008 Mac Pro is pretty ancient. Even a mac mini quad will run circles around it. (except if you need a ton of RAM. Mini is limited to 16GB, I think).
– Cubase is very stable on my 2012 Mac Pro. I don’t think it’s ever crashed or hung on me. I also have way, WAY more CPU headroom than I will ever use (large surround projects @48/24 with lots of plugs). Mac Pro is about expansion, but also more RAM slots than a laptop, which you might want for orchestra VI work. And this machine is very quiet, too!
– You could save money and build a Hackintosh, if you have the time and know-how. Or, you could try a mac mini with your MacBook Pro (you didn’t say which model). Very affordable mac and you could focus on your music rather than the build & maintenance of a Hack unit. Time is money!
Good luck!