It varies a huge amount depending on what program you are using.
Some programs mostly use the CPU and more than a GTX650 provides only a small benefit. So you need to Google the programs using the word "benchmark" to find comparisons.
My quick advice on other parts is:
1) i7-4930K
2) 2011 Ivy Bridge-E motherboard
2) 32GB DDR3 1866MHz G. Skill or similar
3) Windows 8 Pro 64-bit (and Start8 for $5)
4) SSD for Windows and POSSIBLY another one for a video cache (can add later).
*120GB Samsung 840 EVO ($100) for Windows drive recommended. Video cache benefit needs more research.
5) 2TB or 3TB HDD for storage.
Useful read:
http://toptengamer.hubpages.com/hub/Build-Photo-Editing-PC
He mentions spending a lot of the money on the CPU which I do agree on though if knew exactly what COMBO of CPU/GPU the programs would benefit from that might change things.
So you might wish to avoid a Quadro and get a $200 NVidia GTX660 or AMD R9 270X. You'd still need to investigate which card is better for the programs you intend to use.
I hope it helps a little. Again, many video editing programs are still using the CPU most of the time but the question is actually quite complicated requiring some research.
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