Its ALWAYS better to have a C partition for OS & Apps and separate partitions for storage. This way, if the OS gets corrupted, your other data doesn't get affected. I had this happen to me several years ago on a Sony pc but luckily, Sony put 2 partitions on the drives, a C for OS/Apps and a D for storage. I ended up only losing My Documents. This is before I learned that you can & should move the My Documents folder to another partition/drive.
For max performance, Raid 0 will give you better performance. However, you really should look into a separate drive for the OS and run the 2x15k in Raid 0.
I'm assuming that these large files are saved to disk which would be much faster if they were saved to a disk not used by the OS & programs which is why I suggested a separate drive for OS.