<<main question is will the rest of the system stop the card from reaching its potential>>
As you're only running a 1920x1080 monitor there is no point getting an Asus ATI Radeon 5970/GTX 295, without a 30" monitor running 2560x1600 resolution you will never get the full potential of the card at all.
Save yourself some money and get either an Ati HD 5870 or GTX 285/280 instead.
<<Not sure if the WD black drive is worth getting since it's SATA3 and i think the mobo only supports SATA2>>
Buy yourself 1 or 2 SSD from the money saved instead, I have 4 300GB raptors in Raid 5 but still can't match a pair of high performance SSD in Raid 0.
No comment on the X58 & P55 as you'll overclock and you'll hardly notice any performance difference at all.