2 drives are better. And you dont need Raptors or raid to benefit.
The beginning of the disk is the fastest part. You want your OS and your games to be quick loading. If your OS is installed first, then its at the beginning of the disk so its fast, then your game loading wont be as fast, even if they are installed on their own 'games' partition.
Best to get 2 disks... Also for backup purposes, bung your MP3s and family photo etc (the stuff you'd hate to lose) on both disks.
Also, have you ever unrarred/unzipped something on a single disk and its takes forever? Well set your winrar temp file folder to a different disk and it flys along.