I seriously doubt that with a 7200 rpm, 8mb cache hard drive that you will see ANY performance increase by using a seperate drive for audio, especially if you have 1GB of RAM or more. Most everything, OS and application wise, get's loaded into RAM, and once everything is up and going, you are not going to be performing much read/writes from the OS/Application hard drive while you are recording. The data THROUGHPUT of a 7200 rpm 8mb cache drives FAR exceeds the demands of even a 32 track, 24 bit/44.1KHz project would put on it.
This old advice of having a second drive stemmed from the days of Windows 98 KILLING hard drives with it's excessive read/writes and poor memory management. It was PRUDENT to have your audio data (and I would say still is!) on a seperate drive from the OS/Applications because if that drive went to shit, your audio data would be safe and sound.
In the bad ol' days of DAW's on Windows 95 and 98, the OS would just go to hell one day. Something would happen, and it would just be fucked up. Unusable! It is a good idea when doing a OS install to do so on a clean hard drive with a fresh format. Thus, if you had your audio on that drive too, you would need to get that audio transferred to another drive before you could re-format it. PAIN IN THE ASS!
Anyway, ANY modern HD drive is going to be excellent for audio. Data throughput is so high now that you needn't concern yourself with specs on hard drives anymore, unless you plan on doing some MEGA projects (let's say 64 full time tracks or more).
I recommend Seagate drives. Not only do they have very QUIET drives, but most of their drives have a 5 YEAR WARRENTY. No other hard drive manufacture goes over 3 as far as I know. Seagate HONORS that warranty too. They sent me a replacement drive for one that failed one month before the warranty was up. I got a better drive than the one that was being replaced!
I know a guy here who is running BIG projects all from the same drive as his OS.