Re: dual core or 64
For gaming, what you want is the single core Athlon 64 FX. Despite the performance benefits that the X2's extra core brings, the Athlon 64 FX is still the best processor for gaming.
Why? Until games start being coded for multiple processors, you'll get better game performance out of the single core chip. Right now, the FX-55 is clocked at 2.6GHz. The top of the line X2 4800+ is only clocked at 2.4GHz. Both chip's cores have 1MB of L2 cache and connect to the system through a single HT link. If a game is single threaded, it will at any given time be running on only one of the two available cores. So the FX has that 200MHz advantage.
In the meantime, the X2 is targeting the "digital media" apps (or video editing in your case), for which the chip brings a 30-50% performance boost over its single core CPUs.
*Fact: Microsoft Window's Blue Screen of Death vs Computerforums.org's White Screen of Death. Which is worse?