I've ran the greenpower drives in a RAID configuration for a couple of years now in three different setups: connected to motherboard for software RAID, connected to a RAID card, inside of a Drobo. Performance was always the best on the RAID card because it never let the drives go to sleep, so there wasn't a delay while the drives woke up. However, because of this it always meant that power savings came only from lower rotational speeds and not their ability to sleep. Directly to the motherboard worked pretty well, but the spin up times were annoying. I was using the drives to record hdtv and I'd tend to lose the first few seconds while the drives woke up. Finally, I got a Drobo and put the drives in there. Spin up times are really annoying now, but I've stopped using them for primary storage. I just migrate data there as long term storage.
So, no problems with rotational speeds. It seemed that my rather cheap (~$200) RAID card handled it with no problem. But just be aware that you're not going to get the same power savings you'd get on a desktop machine if you've get them connected to a dedicated card.