The white sata ports are usable. So totally 6.
On stock cooler you can probably hit 3.5Ghz. Then you need a better one.
Gigabyte has a 4GHz OC guide here.
http://images.tweaktown.com/imagebank/Core i7 920 oc 4G 2008.12.12_(ENG).pdf
If my Mac Pro is a 10 in noise, this build is a 14.
Mobo has optical out but I have not tried it. I have digital out in system preferences though.
The corsair is fine and you can actually stay at 500w or so with my specs. I just want some upgrade-space.
Not tried firewire.
64 bit apps run fine, but 64 bit kernel has a bug, crasches when you restart or shutdown (maybe not issue for renderfarms)