I do not think anything will be slow, just like that.
There is something called management.
Your system performance will generally depend on primary drive on which OS is there or which is in use for playing games or watching movies.
which drives are in use i.e if you are doing data transfer.
In short no need to really worry.
Rest people ahve already said or asked like PSU wattage(400W good enough for running 4-5 HDDs also with the config you got, you have low power consuming config) and how much each HDD is filled. and no of availability of SATA/PATA ports
Check your power consumption from *www.extreme.outervision.com/psucalculatorlite.jsp
I saw realdan has mentioned about partitions, Varad, partitions are for managing your HDD(not physically but logically), and yes more partitions would decrease performance