looking at the motherboard and gfx card (have only just looked for pics n specs on them), 2 of them should really fit in the top 2 slots, unless you have got a tripple slot card.
You haven't said what make your gfx card is, so can't really look at it and make a better suggestions than what already have.
Also plugging the gfx card into the 4x slot, might make them both run at 4x speed, just so as you are aware of all the possibilities here.