I know I saw a big improvement when I went from 5400 RPM disks to 7200. I think you have your disk allocation bass-ackwards though. Use the faster disk for the data that will get accessed the most - your audio data. It also depends somewhat on how your software utilizes disk space and what sort of work you do.

Whether 10K is a big step up from 7200, I don't know (I ain't been there). As long as there aren't any other bottlenecks in the system, I would expect some improvement.

Also consider the addition noise a faster disk might make and additional heat.

They do make a difference although there is a trade-off in terms of vibration. Unless your case is set up well, vibration=noise so be careful. Heat isn't so much an issue unless your case is small and stuffed with drives. Operating temperatures for most drives are pretty broad ranging these days so if you've got good case cooling then it won't be a problem.

Actually heat is a big issue with today's drives. With 5,400 RPM drives heat wasn't much of an issue. 7,200 and 10,000 RPM drives produce far more heat than than their slower cousins and extra care must be taken to ensure proper cooling. DAW's need to be quiet and reducing fan speed is a great way to acheive that but the reduced airflow can lead to overheated drives depending on the construction of the case. Alot of designs put the drives up high in the case without proper airflow to begin with, slow the fans down and the problem worsens. The better designs have fans that blow directly on the drives and this really keeps the drive temps low even at slower fan speeds. Many gaming and server cases are designed this way and work really well for a DAW.

Another solution is one that David is using in his DAW, the Zalman Heat Pipes. They not only conduct heat away from the drive but they also mechanically isolate the drive from the case so noise is reduced as well. Here's the thread if your interested.

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If you can't comfortably rest your hand on a running drive for a few seconds then it's too hot and you need to consider additional cooling for your drives.

As far as drive speed yes higher RPM drives do provide better performance but the difference between 7,200 and 10,000 RPM's is not nearly as big as the difference between 5,400 and 7,200 RPM's especially considering the price difference. 7,200 RPM drives are sufficient for most users but if you've got the coins and want to squeak out a few more tracks try the 10K's.

Karl is right for most cases audio should be on the faster drive with your programs on the slower drive or better yet both drives could be the same speed. I would suggest a large SATA drive (120 GB or larger) for your audio and a smaller IDE or SATA (at least 40 GB) for your programs. If you have a large sample library you might want to get a larger program drive or even consider a 3rd drive just for samples.

Good Luck on your build