2x150GB Raptor RAID0 Boot drive (~$340)
This doesn't really fit into your budget, since you plan to have a gaming machine but you can try and see how much money remains for graphics.
2x500GB RAID1 backupdrive (size 500GB, redundant) ($180)
2x640GB RAID1 backupdrive (size 640GB, redundant) ($200)
2x750GB RAID1 backupdrive (size 750GB, redundant) ($260)
Only you know how much space you need but if your data is important go with a redundant backupdrive.
EDIT a quick sum:
Option 1: Q9450: $360, 790i $310, 2GB DDR3 $150 : ~$820
Option 2: Q9450: $360, 780i $240, 4GB DDR2 $120 : ~$720
Option 3: harddrives ~$540 (2x150GB + 2x640GB)
Option 4: harddrives ~$430 (1x150GB + 2x750GB)
Option 5: harddrives ~$200 (2x640GB, no RAID)
PSU (750W or above): ~$130 (Corsair, PC Power & Cooling are my top picks)
CASE (Cooler Master): RC-690 ~$110 with shipping
Total without graphic cards:
Option 1 + 3 : ~$1600 leaving ~$400 for graphics (maybe for 1x GTX 280)
Option 2 + 3 : ~$1500 leaving ~$500 for graphics (good for 2x 8800GTS)
Option 1 + 4 : ~$1500
Option 2 + 4 : ~$1400
Option 1 + 5 : ~$1250
Option 2 + 5 : ~$1150