From a safety point of view, if you split each half of your RAID 1 mirror onto a separate channel you're probably OK.
Also, I don't know if the warning is relevant for modern ATA (UDMA) mode disks, or only for disks operating in PIO mode. Note that in my experience Linux defaults to PIO, not DMA, mode because some early implementations of DMA are screwed up. Shouldn't be a problem with modern disks, I run all mine in DMA mode (and Win98/Win2K/NT run ATA disks in DMA). DMA mode's a lot faster if you're not already doing it, higher transfer rate and far lower CPU consumption.
Actually, I don't know if the warning has any statisical basis in fact at all, or if it is just based on opinion or anecdotal evidence!
