Sorry, I can't give you an exact spec on how much faster it is, never bothered to check really lol. All I can say is I forgot to change my rig over from IDE when I first set it up, and then when I changed it to what it should have been in the first place I was amazed at the difference. Trust me, it's worth doing. If you decide to do it, do a few hdd benchmarks before and after the change and see for yourself!
It's not only the speed that's good about it, you can also now eject/swap drives while the pc is running like you would with any external drive.