atm the hd 5850 is pretty much the king of performance per dollar in the high end, I'm pretty sure 90% of the time it'll be a gtx 285.
Also this one card will use far less power then your 2 9800GTX+ it'll have a larger memory buffer. So pretty much any game that uses more then 512 mb will suffer severely on the nvidia set up. The other thing is while most current game support sli and crossfire there are a few that don't so the other card would simply be sitting there doing nothing. And the directx11 is a nice bonus on top of that. I would definitly recommend it, this is coming a guy who has an nvidia card in his rig (gtx 275)
ohh and the suggestion about the spinpoint f3 i would follow too i actually have the hard drive you had on your list 2 in a raid 0 and there great. but these drives have 2 320 gb platters i believe and the spinpoint has a single 500 gb so a far higher date density. High density=greater throughput.
here's a page from an article today they did that included this drive http://www.tomshardware.com/reviews/2tb-hdd-7200,2430-7...
I can tell you from experience that my WD drives do about 80-90 ish per drive from benchmarking so 130-140 is a pretty impressive jump