First you shouldn't overclock to 4Ghz. It is incredibly inefficient because the massive increase in power yields little gain. Second you will never use 12 gb of RAM unless you run multiple VMs. Do you plan to buy a 4890 and then later buy 5850s, because that seems waste of money. Just buy a 5850 when you build. Also that harddrive is pretty slow. Get the WD Black if it's your only drive or get another faster harddrive for the OS. Also if you really plan to have such a powerful machine you might consider the Antec 900 because it will provide better cooling for the massive amounts of heat that your computer will generate.
That is an interesting mix of components, and although everything you listed should work together, you might consider a more balanced mix. For instance, the motherboard, CPU, RAM, and PSU are all excellent choices, but you combine these with an entry level case, a mediocre hard drive, and an older generation video card. If cost is driving these decisions, you might consider a P55 motherboard, i7-860 CPU, and 750TX PSU. This should save you about $150, and that could be used to upgrade your case (Antec 900, HAF 922, or other), your hard drive (Samsung F3 or Caviar Black), and video card (5850). You will also need a decent heat sink fan (Hyper 212, Xigmatek, Noctua).
By the way, the i7-860 and i7-920 are essentially equals when it comes to performance. The i7-860 is slightly faster in some tasks than the i7-920, and visa versa (see Anandtech.com for benchmarks).
Good luck in your build -- you are on the right track!
As for the HD, the WD Black 2 TB model's performance can not be compared with the rest of the Blacks. That one has 500 GB platters and the others have the older 333 GB platters which are therefore slower. If not getting 2 TB, grab a Samsung or Seagate 500 GB per platter drive.