I'm using an MSI motherboard which has a Promise Fastrak Lite RAID controller onboard. I have twin Seagate Barracuda 80Gb drives connected.
It is limited in that it only supports striping (aggregates both drives for around 160Gb, but no security of data), and mirroring (I only get 80Gb capacity, but if ONE (and only one) drive fails, I don't lose my data)
I have found this drive performs very well as a repository for MP3s, especially as they are streamed across to another machine connected to my loungeroom stereo system, so I don't think performance is a major issue here.
I have already filled my 80Gb mirror, so my next plan is to buy a full-blown Promise PCI RAID controller, which will allow full RAID (ie. then I can buy one or more drives to add to my existing two, and create a RAID 5 array, where the data is striped across the disks, but it includes parity info so that the data can be rebuilt in the event of any drive failure)
I don't think should really be an issue here. I mean if you used the built in RAID controller drives to house a hard-hit database, you might notice it, but not for storage of (fairly) static stuff like a bunch of MP3s.
Hope this helps...