Hi. If you had a 250GB drive and a 500GB drive, could you use them together as a RAID 0 on a Mac Pro if you partitioned the larger drive to equal the smaller one?
You could then use the spare 250gb of the larger drive for something else.
I successfully used two 1TB identical drives in both a RAID 0 and RAID 1 configuration at the same time.
I split them both down the middle (well a 80/20 split). Did a RAID 0 on the two outer most partitions for video editing and a RAID 1 on the inner most partitions for backing up photos.
I wasn't sure how two different makes/sizes of drives would work. It would be just a cheap option until I can afford two larger identical drives.
look at softraid (http://www.softraid.com/). With that you can run split a disk into chunks and use different chunks for different raid levels. you can even link disks using different protocols (firewire, sata etc). Two of my disks are different sizes and I have the first sectors of both combined into a stripe array with the remainder left as normal storage. It lets you do exactly what you ask.