I think some of you may know about this already.
There is an open-source (meaing free) simulation engine out there made specifically for space flight simulation. It's incredibly accurate in terms of physics, and is highly mod-able with all manners of spacecraft, real or imagined, historical or futuristic.
The hardware requirements are modest as long as you're running something less than 5 years old you'd be fine.
Prepare for a steep learning curve! But also prepare for liftoff.
http://orbit.medphys.ucl.ac.uk/orbit.html
