Specifying Render Parts

I’ve recently rendered a work in parts because I only have 512 mb of ram which seems to be choking the process. I think it’s helping a bit, but that’s all irrelevant.

I want to know if there is a way to tell Blender to render specific parts or in a specific order. With this you can technically “pause” a render by cancelling it. Save the half-rendered file, render the rest later, then splice them together in Gimp or Photoshop.

Why would this be useful? I had a render going on for 8 hours and it only managed to finish 2 of the 16 parts. The deadline was approaching, I did the math and it didn’t look like I was going to make it. So I changed the settings, blah, blah, blah, and rendered a new file. It would have been nice if those 8 hours weren’t wasted, that’s all.