The 7900gt is good but the mid range dx10 cards are coming out early next year. You should get an evga to hold you over until then so you can trade it in for a dx10 card and then you should be set to play future games that will require dx10 (i imagine there will still be dx9 versions of the games, but playing them on dx10 would be better).
Of course you will need to get windows vista to take advantage of dx10, so keep that in mind as well.
I can see why you want to raid 0 your storage now. I still think 1 TB is over excessive for a couple hundred movies (unless they're DVD rips).
CPU: e6600 @ 3.2ghz
RAM: 2GB G.Skill HZ PC6400
Mobo: ASUS P5B Deluxe
GPU: Sapphire HD4870
Soundcard: Creative Labs X-Fi XtremeGamer Fatal1ty
HDDs: 250GB WD + 750GB WD + 2x1TB WD
Monitor: Samsung 226BW
PSU: OCZ GameXStream 600W
Case: Cooler Master Stacker
OS: XP Professional SP3
Mouse/keyboard: Logitech MX518/Saitek Eclipse II
Speakers: Logitech Z-2300