Adaptec makes a hardware IDE RAID card, and It rocks. But it's like $450 it will however cause your disk performance to be extremely good.
Promise is the best Software IDE RAID card, but SW RAID kind of blows...
If you are using RAID1 (mirror) for data saftey that's different, but because of the CPU overheard involved with SW RAID it doesn't tend to help performance very much except on very specfic types of things. If you have alot of CPU time to spare, and you are doing extremely disk limited things then RAID0 might help some...but for most stuff it's pretty useless.