Here's my opinion:
Go for the Quad with an upgraded graphics card. As more and more app developers support Open CL, a good graphics card will be your most cost effective performance boost. The things that Open CL will really excell with are going to be exactly what you're going to be doing (video editing/transcoding and bitmap pixel pushing. As Photoshop and Final Cut Studio are updated to off load to the GPU you may not notice the lack of 4 extra cores that much.
Also, the poster that suggested buying a new computer in 3 years is really on to something. Macs really hold their value pretty well compared to other computers. Try looking on ebay at 1st gen Mac Pros (3 years old) to see what I mean. If you take good care of your machine and sell it before it become ancient can really offset the price of a new one. If you really think about it, this may be more cost effective then running the same machine until its worthless(6years) and than buying a new one.
Also, you're obviously concerned with cost but whatever you do, do not skimp on Apple Care. I don't know if you've see what a logic board or replacement Xeon costs but its well worth the gamble to get apple care.
Sorry if these things have already been mentioned, I didn't have time to read every post. Hope this helps