A hardware RAID5 card will be much faster than the software RAID option. You can also get hot-swap enclosures for hardware RAID cards which you wouldn't be able to do with software RAID.
I personally would use a 3Ware Escalade IDE RAID card but they're much more expensive than the Promise cards. The 3Ware cards have much better support under Linux and I've found them to be more reliable. I've got an 8 channel Escalade 7500-8 but it was around $500. Not sure how much the 2 channel one is though.