There will be a benefit to using two hard drives if you put your pagefile and your personal "My Documents" folder on the non-OS drives. For example, when FireFox starts up it accesses the OS drive and your personal files very heavily (to load image cache, browsing history, suspect web sites, etc.). If those folders are on separate drives then there will be less contention and better performance.
But the downside is that you'll have more work to manage the files. For example, if your data drive fills up and you still have free space on your OS drive, you'll have to decide what files to move (hopefully ones that aren't often used) and then keep track of what files are where.