2ssds in raid 0 and a harddrive to back up?

You already stated you were using raid 0. That would work fine for incredible speed. There is however no recovery if a drive goes down. Just attach the other drive to another SATA connector and don't put the HDD into the raid. This will show up as a non raid device and work fine.

Well you only have 256 GB of storage, so you only need a small hard drive, maybe a 320 or 500. If we knew what you had for a motherboard we could look further into it, but you could use Raid 0+1, but that's not that common. Also, you can't migrate into a setup like that, you'd have to start over, which you said you already have the SSD's in Raid 0.

In your case, I'd simply add the harddrive, and set up a scheduled backup job in windows, and do it as an image, since it's only for catastrophic failure.