Galaxy simulations just rock don't they
I used to develop a galaxy simulation program some years ago, just for fun. I once had my computer running day and night for a month, to complete a 1.000.000 particle simulation. There are techniques to reduce the number of required calculations, so the calculation time doesn't grow exponentially (so 30,000 stars don't require 900,000,000 force calculations), it's of course a lot more complex to implement than a brute force O n^2 algorithm.
Here are some screen shots from my program and a 400,000 particle simulation made on my old Athlon 1800+:
http://www.titirangiobservatory.com/galaxymerger.html
Note: I'm not sure the executable works properly because it has been years since I last fiddled with it.