Hi Taivo,
While software RAID-1 is your cheapest option by far, if data security is your main concern instead of performance, then it really does not matter too much (unless the option you choose causes the server to crawl - haven't seen that happen in either hardware or software RAID setup so far).
I did a Google search as Charlie suggested and I'm not convinced. I don't wish to get into the debate of whether the Highpoint / promise cards are not "true" hadware raid controllers (I've also come across comments that there are no such things as true "software" RAID

as well some time back). One of the sites I was reading searching Google was highlighting a problem some were having trying to use an Onboard RAID of some motherboard to achieve RAID 0+1 (striped and mirrored - a setup NOT supported out of the box with SME's software RAID procedure anyway), they did not mention the fact that most motherboards use cut down (or "lite") versions of their controller chips for integrated solutions and these integrated chipsets are usually not directly supported by the chipmakers.
My own experiences with Highpoint 370 Based cards in SME servers have all been positive ever since I compiled the drivers for them (from Highpoint's web site). While most lower end cards don't support hot swapping, higher end cards (like Adaptec's 2400A do). If you use software RAID, you have the situation where you will need to either :-
1. configure each drive on separate channels
or
2. configure the drives in a master/slave setup on a single channel.
If using (1), and assuming you have no additional IDE ports on your system and you have an IDE CD-ROM drive daisy chained to one of the drives, you will slow down the drive that is connected with the cd-rom and thus your RAID as well, If using (2), If the master drive fails, depending on the nature of the failure, you will sometimes (very often) find that you have no access to the slave IDE drive either until you remove the dead drive and either replace the dead drive or reconfigure the slave as master (a major disadvantage of IDE as opposed SCSI).
One additional benefit of using a hardware RAID card is you can use them on older motherboards which may not be able to take advantage of ATA66/100/133 drives. The onboard controller of these cards will (depending on version of card) and you can then have better performance as well as protection of mirrored drives over what you can get with just software raid (no matter how efficient your Linux drivers are !).
In any case, a UPS and a tape backup system is probably more useful than both software and hardware raid. If your server shuts down badly (power outage, etc.) there is a GREAT possibility of filesystem corruption, some of which are NOT fixable by fsck, and it won't matter if your drives are mirrored or not, they BOTH get corrupted (been there, done that more often than I care - here to waiting for SME 5.6 to be released). If your drive dies, replace it and restore from backup. True, there will be down time, but if you can afford it all, have it all, UPS, tape and RAID !

Kelvin