Most Essential Upgrade for Video Editing

I am a student video editor and want to upgrade my Mac Pro (Early 2008), upgrading either the Processor or RAM first (eventually both) but I do not know which is more essential for what I want to do. So here are my purposes for my computer use: I want to be able to edit HD video with programs such as Adobe Premiere CS6, Avid Media Composer 8.1, Photoshop, and Aftereffects that includes me engaging in processes such as rendering effects, compressing large video files & importing and converting media smoothly with minimal or no crashes at all. My spending budget is approximately $600 so I want to use that as efficiently as I can. I know when buying certain pieces for a mac it is very specific for what kind of piece you can buy so if you guys have any recommendations for device specific hardware also please let me know.

Also here are my system specs:

Mac Pro (Early 2008)

Processor 2 x 2.8 GHz Quad-Core Intel Xeon

Memory 8 GB 800 MHz DDR2 FB-DIMM

Graphics NVIDIA GeForce 8800 GT 512 MB

Software OS X 10.9.5 (13F1066)

Model Name: Mac Pro

Model Identifier: MacPro3,1

Processor Name: Quad-Core Intel Xeon

Processor Speed: 2.8 GHz

Number of Processors: 2

Total Number of Cores: 8

L2 Cache (per Processor): 12 MB

Memory: 8 GB

Bus Speed: 1.6 GHz

Memory: 8 GB

Model Name: Mac Pro

Memory: 8 GB

Bus Speed: 1.6 GHz

Model Name: Mac

Memory: 8 GB

Bus Speed: 1.6 GHz