Do you have a monitor already? Do you need the hard drive bays in the Mac Pro? Are you okay with the size and extra power usage on a pro?
I bought a used 2009 8 core Mac Pro and I love it. I probably wouldn't pay $600 for a 2008, but it is compatible with OS 10.9 so that is a plus. Overall...
If you have mkvs encoded in h264 you can easily mux them into mp4 files using subler on mac. This takes very little time (~5 min) and it is not reencoded so quality is preserved. Unfortunately there is not a great windows alternative but you may be able to find one. I prefer this method over...
I own the Crucial M4 64GB ssd and use it for my parent's computer which has practically nothing installed and I still feel it is too small. I would not settle for less than 128GB in my computer. I would keep check on current deals and I am sure you can find one for a decent price.
I liked Firefox and still do on Windows but they have poor support for OS X features. Chrome is just easier for me to use on a Mac.
Firefox needs:
-Full Screen through Lion API
-2-finger forward/back swipe
Sounds small but it can make a big difference on a laptop.
I have always heard Intel motherboards are really stable. I chose to avoid them because you cannot overclock Intel boards as easily, but that doesn't seem to be an issue here since you cannot overclock CPU on the H67 anyway.