Does it do that during idle or when the harddrive is doing something?
Try tightening the screws that secures the drive to the case. If it is still vibrating badly, you may want to start shopping for a new drive before that one craps out on ya.
My hard drive hard drives so bad it literally shakes my case is this bad??
Back up your data, first of all. (If nothing important, then no worries.) Download seatools, do a full diagnostics test. That drive should still be under warranty, but you have provided rather limited information.
a 250 GB seagate should be a single platter, probably a 7200.9 or possibly a 7200.8. The vibration probably means that there is a spindle bearing going bad. Seatools will probably not find this type of failure, as it is more geared towards platter/head integrity.
Again, you neglected to provide whether it was IDE, (I assume) or SATA, and whether you could use an SATA drive on your MB. Seagate still makes both interfaces, but IDE's are getting more expensive per GB than SATA's.