Buying this €300 part would mean going a bit above budget, though. Do you reckon there will be a large difference in performance using the raid card. I guess it would mean that there would be many more possibilities in terms of choosing a motherboard.
I know your personal quote says you're done helping people with AGP and P4's, but as I would not need a motherboard with a raid controller, could I not use my old P4 pc with a 3.0Ghz processor and plop in the raid controller + 6 hd's?
You should first determine what OS you plan on using as the OS is what will really determine your hardware requirements. There are a few choices like; FreeNAS, Openfiler, Ubuntu, or Windows Home Server.
FreeNAS is a great NAS OS with minimal hardware requirements whereas Windoze Home Server has the hardware requirements of a regular pc.
Also, a hardware RAID card is the better option over mobo RAID especially if you plan on using RAID5. ANd yes, there is a noticiable performance difference between the two. If mobo RAID5 then you require a more powerful cpu as opposed to a cheaper/less powerful cpu with a hardware RAID ontroller due to offloading the parity calculations to the controller card.