RAID 0 can (under optimal conditions) double the transfer rate, but it won't improve the access time. Access time is more important for typical tasks such as booting and starting up programs, so RAID 0 won't give you all that much of an improvement. Expect performance for those kinds of tasks to be something in the neighborhood of 10% faster. 10% is measurable, but not noticeable.
If you want significant performance improvements for those kinds of activities, you need to use drives with an inherently faster access time such as 10KRPM Velociraptors (2-3X faster access times) or SSDs (100X faster access times).