[amcd]

More hard disks, bigger hard disks, faster hard disks, different RAID level and ultimately all disk based solutions. If you are heavily disk dependant, you are doing something wrong anyway. RAM and more RAM will help you. Disks are for storage. They should not be accessed for normal operation. Anything which is accessed frequently should be in RAM, and that includes php files, css and js files, buttons, smilies and icons, most of the database and even the avatars and signature pics of the most active users.

If you are running a php tracker, think about moving to a better tracker.

Once you outgrow your server, shifting from single disks to RAID will not do you much good if you already have loads of RAM and are not accessing the disk every other second. The only path then is to move the database to a separate server.