RAID mirroring of hard drives slows FS much?

My new machine has 2 SATA hard drives which I have set up as mirrored drives. (It wasn't intentional, but that is the way it ended up after problems I had getting Windows to find hard disks to install itself onto in the absence of any IDE hard disk on the machine).Presumably there is a FS performance penalty in using two drives in this way, compared with using just a single drive, as the CPU has to do more disk writing when it should be improving the FPS :). Any idea how much of a penalty?(I know I could theoretically get performance gains by operating the two drives in a striped configuration, but I don't want to take the additional failure risk).--Bryn