Okay then - did some additional searching on power issues in this forum and the clue is: I'm not upgrading to 4 drives, because the standard Snap 2000 PSU won't be sufficiant to power 4 bigger drives (e.g. 4x 400 GB).
I found a PSU delivering enough power (I think) at a german shop (
http://www.pollin.de/shop/detail.php...=Njk1OTc5&ts=0) at a reasonable price. But the PSU is too big to fit into my 1 HE 19'' housing I plan to use (if I also put 4 drives and the mainboard in there).
So I'll have a try and only upgrade with two bigger drives and hope the PSU will manage those and some additional fans I ordered to improve airflow in the 19'' housing.
Thanks for the help to all!