Just getting a SATA hard drive will give you a small boost in general as from the numbers SATA drives perform better than their IDE counterparts. But it's not a big difference and not worth upgrading to. What is worth upgrading to is a faster drive. If you currently have a 5400rpm or 2MB cache 7200 rpm hard drive you should definitely think about getting either an 8MB SATA drive like the Seagate 7200.7, the Samsung SP80 (be careful not to get the 2MB cache version), or the Desktar 72K50. But the real boost would come in the form of a WD Raptor SATA drive. These are 10k rpm drives. But the only thing these drives will do is make Windows feel "faster" and more responsive and decrease level loading times in games. They won't make the actual in game portion any faster, that's mainly CPU/RAM/Video card.
Personally, I love my Raptor because it makes Windows feel ultra snappy and fast
and because it massively decreases level load time in games like FarCry. If you have an 8MB cache 7200 rpm IDE it's probably not worth upgrading unless you're crazy like me
Check out
www.storagereview.com for reviews of hard drives. They do really great tests with excellent comparisons and the forums are just untouchable; a good group of well informed smart guys who never fail to give the proper advice.