I have a PC with an intel i7 920, 8 gig of ram, Nvidia gts 240. And Use a system drive 600gb, and I have 2 1.5TB hard disk. I use 1 of the 1.5 TB for video files and the other is for backup.

I was thinking about getting a 3TB dive for backup and turn the 2 1.5 TB drive into a Raid 0 does this sound like a good idea to improve performance?

Yes, that would be a better set-up.

Generally speaking, yes. However, you might want yet another hard drive (in addition to a 3.0TB hard drive) if you're going to use a RAID 0 with your two existing 1.5TB hard drives. This extra disk will serve as your project files and/or output disk.

And speaking of 3TB hard drives, there are only two that spin at 7200 rpm: a Hitachi 7K3000 and a Seagate Barracuda XT. The other 3TB hard drives spin at only 5400 rpm or 5900 rpm.

How much space do you end up with on those 3TB drives after formatting?