I've had an ibook/macbook as my sole computer my whole time in law school (one exam to go and I'm done!!) and it's worked quite well. Since you already have the imac, though, it sounds like you made a good choice.
Oh, one more thing - although you're right to save money where you can... go for the netflix!! Totally worth it.
Netflix... so... tempting...
Good luck on your last exam, and CONGRATS! If you have any law school advice (tech-related or otherwise), please shoot me a PM. I'm pretty much terrified.