ok lets take this one at a time
no more cooling problems with AMD
infact pentiums are hotter right now ... tho even thats fixed to some extent now i think
k8n is if im not mistaken .. an amd board ...
939's are a better socket .. but currently from asus only the VIA chipset is available ... dunno when the nForce4's will be out .. those will be the best ...
64FX-53 will be more costly than the amd64 so look into that as well
price difference would be there acc. to ur selection
if ur going for the most high end of components ... like i already mentioned. .. there will be a diff of upto 5k or more ..
yes 9800 pro is an agp8x card .. and it is supported by amd MOBO's
the processor has nuthin to do with it

dual channel ddr 400 2*256 mb is just perfect for now