Hello, I am new in custom router building and have several questions to ask about RouterBoards.
We have at home large network 25 Gigabit devices and over 15 wifi (g/n) devices and our main router just keep crashing every 10 minutes it reconnects :/ I think it's too much for him to handle over 40 clients simultaneously. Current networks looks like this

ISP - Dlink DIR-655 (Working as router and access point) - Clients with 3 gigabit switches...

I was looking at RB800 but it has only two gigabit ports because I want to replace DIR-655 it has 4 ports. It handles all work (torrents/files transfer(backup)/gamings/dlna/etc..) I think maybe RB800 is bit too powerful?

it all depends on how much traffic is actually passing though the router. RB800 is quite powerful device that can be as AP itself (4 miniPCI slots for cards) and 3x1Gbit ports.

If you state what you are going to do on the router, and what are bandwidth requirements then some forum user could advise on what router to have.

Also, you can check RB450G or even RB750G/RB750GL