yeah, but is 2 x 1,5T (sata 2) on raid 1 will be enought to handle 100 ~ 200 clients??
It can be, but it is all relative to the type of plans you are allowing and their usage patterns.
As to HD's and shared environment hosting, I would invest in RE/Enterprise drives that are at least 7200 rpm and go with 4 x XXXX in Hardware RAID10 at least, dual xeon processors and loads of RAM, we don't do less than 24 GB per shared node, but we also specialize in high usage plans and don't get anywhere near 2-300 user accounts packed onto the servers either. You don't mention plan type, but if you are planning on being the next HG/BH offering "unlimited" packages then I would really start researching into all of this more and you should go with something like Cloudlinux as well.
One final suggestion, have a backup service in place.