I don't want to make any assumptions (this could be a fine art photo piece in a gallery for all I know) but I would be surprised if you really want or need to print at 150ppi for a piece this size.

As you say, your image would need to be about 1 billion pixels (1000 megapixels). Is your source imagery going to provide this? Are your audience going to notice the difference? You say "consisting of photographic elements" - only?

Also, there's no point in upgrading your PC and providing a GB-sized file if the printer's hardware can't handle it. I would make absolutely sure of this before going to these extremes.

If the main thrust of the question is "how do the pros do it?" then the answers to http://graphicdesign.stackexchange.com/questions/487/what-ppi-should-a-large-format-artwork-for-print-be-done-at should cover it.