RAID 0 means if 1 drive fails, you lose all your data. Thus having 2 x 500 GB drives in RAID 0 means you double your chances of losing your data compared to 1 x 1000 GB drive. RAID 1 gives you parity, but you get 1 drive's capacity (i.e. 2 x 500 GB in RAID 1 gives you data parity, so if 1 drive fails, you don't lose anything, but you'll only get 500 GB of storage).
The DFI board I linked to has 6 x SATA2 ports, so it leaves you room to add 2 more hard drives without having to purchase a SATA controller card. It has 6 USB ports, though it doesn't have Firewire (~$10 card to put in, but if you want Firewire onboard, there are other boards that offer it for roughly the same price).
The 8800 GT won't help much, if at all, w/ CAD or CS3 unless you're doing ultra-high-end 3D rendering using CAD (in which case you should be looking into an ATI FireGL or Nvidia Quadro). CPU, RAM, and HDD is what is going to help there. If you plan on playing casual games, I recommend the ATI HD 3850 256 MB, which is $100 after Mail-In Rebate. It'll play every game fine at 1280x1024/1440x900 on Medium settings or better, including Crysis. If you plan on playing at higher resolutions, then the 9600 GT ($145), HD 3870 ($155), or 8800 GT ($165) will be a better fit, though obviously more expensive.
