Here's an excerpt from a post I made in this thread Question on huge size images

Quote Originally Posted by Natai View Post
The image is 20122x11634 @ 450 px/in. The PSB file is about 3.7GB saved, and PS consumes around 7.75GB in memory while I'm working with it. So tilt's point is a good one - go with 2-3 times the amount of RAM relative to the saved file size.

So, bottom line if you're working with files larger than about 1.5GB, you will probably benefit from upgrading to 64-bit. Just be sure to keep in mind what Redrobes posted - you're entire system has to be 64-bit, the CPU, OS and the application.
That thread might be useful as it contains some good information about RAM, memory pointers, etc. And just about all the posts are referencing maps at the scale you were talking, 20k x 10k and up.