Upgrading RAM in Late Model 2009 iMac
You have the perfect test setup here:


  iMac late 2009 model ... to export a 3gb video file in Final Cut Pro it takes 1 - 1.5 hours


You can restart your Mac and quit all the apps except FCP and open activity monitor. You can watch the memory usage during the export. If the memory pressure is green, then no amount of RAM will help speed up the export. 

Assuming RAM isn't the bottleneck, next to eliminate is the storage - watch the iops and data rates for read and write. That too is easy to benchmark - ssd gets higher iops than most single hard drive. The other answers have great advice and your bottleneck is likely CPU/GPU but you can measure precisely what you need with activity monitor.