I've used RAID 0 with both 7,200RPM and 10,000RPM drives and the performance increase over an independent drive is nonexistant. Simply put, if you were to use a machine with a single 74GB raptor side by side with two 36GB Raptors, chances are you couldn't tell which is which.
10,000RPM drives on the other hand are significantly faster than a 7,200RPM RAID 0, however I will admit larger capacity 7,200RPM are getting faster. Look at my computers, I've been using 10,000RPM drives for years and I simply will not go back to a 7,200RPM drive as a primary boot drive because there is a huge difference. Raptors are good investments because they almost never fluctuate in price, which is rare for computer hardware. You do not buy them for storage, you buy them to speed up load times on programs.