Hey there, delobe!
I don't think you'd have anything to worry about when RAID-ing two HDDs in the SATA 2 ports. Even the fastest mechanical drives can barely reach the speed of 200 MB/s, whereas SATA II ports are able to transfer up to 300 MB/s. So, yeah, you won't suffer any loss of performance by connecting your array to the SATA II ports on your motherboard.
Just remember to back up your RAID 0 configuration, because it's one of the most vulnerable arrays! If one HDD fails, you'd lose the data stored on both of the drives, so be careful!
Good luck!
SuperSoph_WD