All motherboards are much the same when it comes to what your running on them, so a cheap board will game and edit pretty much the same as an expensive board. E.g a $50 nf4 board speed wise will almost match a $120 nf4 board at stock.
There isnt anything such as a game board, motherboards are motherboards. They vary in chipset and features but they all do the same core things.
What you pay the money for on expensive ones is better quailuty components, better oc options and potential and more onbpard features such as firewire.
Overclocking can help speed up encoding but will be of little use in editing apart from just a generaly quicker system.
Graphics card wise, depends on the budget but as your not a big gamer maybe a 7600gt or x1800gto.
If your not going to sli, save the money and get a single slot motherboard something like the ASUS A8N-E NF4 ULTRA