Let me start out by saying that I don't think this is worth it in the slightest and I don't think you'll notice any difference. With that out of the way, I'll answer your questions.
Copy all your media off the drive onto your backup and use a gparted live cd to shrink your OS partition right down to whatever you want.
I don't know how to guarantee that it's on the outermost section of the platter, but I'm not sure it'll make that much difference to be honest. With gparted you can move the partition to one end or the other of the drive, you'll just have to benchmark both.
The speeds you'll achieve aren't even remotely comparable. If you're very lucky you'll get a x2 performance increase in iops alone. An SSD would be more like a x100-x200 performance increase.
See here for more info, note however that they are using 10k or 15k drives in RAID 0, your drive will be far, far slower than that.