GX2 is essentially 2 8800GTs. And as much as i love Asus motherboards, i heard that p5n-d isn't very good for overclocking but if thats your budget then i guess you would have to go with it, i would recommend 750i FTW from EVGA since i've heard some good stuff about it but i aren't really sure. honestly, by all means, try the asus, usually Asus motherboards yeild pretty good overclocking results. I would go with a 9800GX2 if you can find one for about $300, if not, you can go with a 8800GTS 512mb. PSU, HX620 from Corsair but that just might be a bit tight and to give it headroom, you can get an Antec Quaddro 850W. Both are modular so less cable hassle for you.
That should work since p182 is about to support 8800GTX cards so it should fit most cards, i am pretty sure about that. If it doesn't, just take out that Hard drive cage and you are sitll elft with the one on the bottom.