Raid 10 is faster than Raid 0 for the OS and general applications. Raid 0 increases Random Access times with each drive added whereas Raid 10 decreases Random Access times. The OS does a lot of small random reads and writes so having faster Random Access helps more than throughput. Anything over 150MB/s read/write does not provide much, if any, boost in OS performance. A 2-drive Raid 1 has shown to provide more benefit with an OS than a 2-drive Raid 0. This is possible because drives have very high throughput and Raid 1 uses a nice feature called "Split Seeks" where the OS is able to search both drives simultaneously.
I have compared 4 150GB Raptors in Raid 0 vs Raid 10 and the Raid 10 was faster for the OS & Apps. I now have 4 74GB Raptors in Raid 10 for the OS+Apps and a Raid 0 array for Adobe stuff. One of the 150GB just died so I moved the other 3 to my 3ware Raid controller in Raid 0 for video editing.
With the 4 150GB Raptors in Raid 10 for OS+Apps, I had also created a Raid 5 array for storage. I can guarantee 450MB/s is not possible with even 6 drives in Raid 5 except for the initial few second burst which is only due to the Write Cache. I was lucky to get more than a constant 40-50MB/s write speed and up to 100MB/s read speed.
However, with 3 500GB 7200.11 in Raid 5 on my 3ware controller, I get 150MB/s write and 160MB/s read speeds.
Using the Intel onboard Raid for Raid 5 is playing with fire. There are a few long on-going threads here about problems with Raid 5 and Intel Raid. Look for Shadowflash or Sub Mesa's posts as they have extensive knowledge on the subject.
Even SomeCallMeTim has problems with the Raid 5 needing to rebuild after a BSOD. Another extremely important aspect to desktop Raid 5 and using desktop drives is that the drives don't have Time Limited Error Recovery. When a drive takes more than a few seconds to respond, the Intel Raid controller will "Drop" the drive from the array meaning that the Raid 0 array is now toast and hopefully the Raid 5 array can be rebuilt. I had this happen with 4 7200.10s in Raid 10 for OS and Raid 0 for other stuff. The drive took too long to respond so it was dropped and I lost 200GB of data on the Raid 0 array. This was before I ever knew about TLER. This is also why I use enterprise class drives(Raptors) for the onboard Intel Raid.
PS If you are using Vista and/or plan on doing more than 1 thing at a time, Raid 5 will be horrendously slow. The Raid 5 array with my 4 Raptors was EXTREMELY slow when the OS(Raid 10) was doing something or I was watching a movie from the Raid 5 array. Also, Vista constantly reads and writes which slows down everything.