The raptor is big enough for the OS and program files, it's also faster.
If you keep your personal files like photos, music and video on the 250gb you will be ok. You can also install programs onto the second drive if needed.
I would keep 25-30% of the primary (OS) drive free. You can fit a lot of programs onto 40gb give or take a few.
When using video edditing and burning dvd's make sure to set the default folders to the second drive, as they tend to shove everything into the document folder which can chew up hdd space.
Doesn't make a difference, really. Usually you want to install the OS on the fastest drive you have available... but if space is a concern (which it shouldn't be), then installing on the larger drive isn't going to hurt much.