Main Reason: I'm not made of money
Sub Reason: I haven't had any experience with AMD since AMD Athlons. Since then I have always had an Intel processor, which I have always known to be the best for video processing. The main reason for my new computer is going to be video processing, not gaming (even though there will be occasional gaming). I have always seen each of processors as follows:
Intel = Mainstream Application / Video Processor
AMD = Mainstream Gaming Processor
Now, if any of you has had experience with using AMD's X2 processors for encoding purposes (mainly DV -> MPEG-2 via AVISynth), then please let me know. I want to make sure my hardware is as good as I can get it.
Also, Intel provides Hyper-Threading which is definately an advantage when it comes multi-threaded applications (ie: video encoding). Four vs. two threads being simultaneously processed by the processor sounds much better to me (or course, I am talking about a Pentium D with hyper-threading enabled).