Raid 0 speed question

Hi all, For those who have lots of experience with this: I need more hard drive space and a faster disc too. Assuming software based raid is reliable enough for a home user, and money is very tight, I have two options: 1. Buy a new fast disc 2. Use kernel software raid to mirror the current Western Digital Caviar 5400rpm 2mb cache 10gb drive with a 10gb partition on a new (but now very cheap) 5400rpm 40gb drive. The remaining 30gb on the new disc would be /home Which setup would be faster? Thanks Hans