Bla!! is correct. a faster hard disk will load the game faster and make your applications like Word, powerpoint, photoshop or Flash faster. it is useful for applications only because you do a lot of reading and writing to hard disk. it will not make games run faster, the video card is the biggest factor for 3D games. I play yuri's revenge which is CPU depedant since its 2D. that game replies on CPU more than video card.
to clarify about IDE interfaces. IDE hard disk have 66/ 100/ 133. this is measured at megabytes per second. this is the maximum transfer speed, not the average as didi says. it almost never reaches near the maximum transfer speed. interface doesn't tell you hard disk speed by itself. you have to consider cache, RPM, response time , ect. anyways...look into video cards if you want better performance for 3D games