go with the regular A64, the fx costs a lot more money for a little more performance
If you want to go for a dual
cpu system you will need to use opterons and a s940 board and ecc memory, in other words very very costly. the opterons are meant more for servers and high performance workstations and not really the consumer
I would recomend going for a pci-e board as agp won't be around much longer and you'll pay premium for it
LN2 cooling is really not practicle, overclocking sites only use it to set records overclocking and it's not a permanent solution, they probabally destroy the hardware using it