You can use an 80mmx80mm for both the front and the back. I would get 2x of whichever fan you get, as that one in the front is pretty worthless. You can use a fan with 3 or 4pin headers. If you get a decent 80mm fan, you should probably get one with a 4pin header as people have had the 3pin headers die on em when using higher rpm fans. They have pass thru headers so you won't need a free 4pin header, just make sure your PSU is up to the task.
Chiz