I'd strongly suggest getting another GB of memory.  I did a 12x16 @300dpi render in 16 hours on a single computer.  That was using the user render settings and turned fairly high since there was lots of trees and grass.  If you have water or lots of reflection in a scene, plan on spending much longer on rendering times.  

Network rendering is the only other way to speed things up.  Look around and see if you can get some older systems - they don't have to be fast systems or even match each other (you can mix PC and Macs). 

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