Best Way To Improve Video Editing of Mid 2009 MBP

Hello, I have a MBP that I would like to improve to get better performance out of for video editing of a GoPro Hero 3 Black. The camera is the latest version and capable of taking some very high definition videos. Initially I plan on using GoPro Software and/or iMovie. Currently, working with the videos is very slow. The specs on my machine are:
Mid 2009 MBP with 2.26GHZ processor, 4gb ram, and 160GB HDD and running OSX 10.8.2.

My question is, I want to improve the ability to edit videos and not sure where the best bang for the buck is. Some say RAM, other say HDD (external or internal, not sure)

I am sure folks here have crossed this bridge before. What have you found worked best?

Thanks!
DAVE

Thank you. I actually just ordered 8gb of ram ( max it can take) from OWC. When adding an SSD.... Just go with that or that with an external drive? My 160gb drive is currently about 66% full