Faster OS: 1 SSD with your OS, and the other drive with your data.
Faster massive file editing: Two Caviar Blacks in Raid 0
Better: Depends on what you want to do and what your priorities are, plus your backup strategy and how much work you can afford to losw.
For faster OS, the SSD is going to be faster than the Raid 0, and a lot less work.
If you work with huge amounts of data, like video editing, your limiting factor will be how fast your data drive is, and the Raid 0 will serve you better (actually, raising memory to 8 GB or more may serve you even better).
RAID 0 is twice as vulnerable to drive failures and much harder to recover data from than a single failed hard drive, so I advise against using it unless you do regular backups.
Now the easy question: my choice. I run Win7 on an SSD with large spinning magnetic platters for my data.