No, you don't get a doubling of performance, it doesn't work like that -- any more than if you have a car that can go 120 MPH, if you put two motors in it, it can go 240 MPH.
In fact for most single user applications, the overhead of the RAID processing will be higher than the potential speed gains.
RAID 0 (or other forms of RAID striping) is mainly useful in two situations:
1) Multiuser servers
2) Handling massive, sustained data streams, such as capturing HD video
RAID 0 is also horribly risky, an error on one drive will kill the data on both drives.
Raptors run hot and noisy. And because they are small drives (both in capacity, and the physical diameter of their platters), they drop in performance rapidly as they fill up, and start moving to the inner tracks of the drive. The outermost tracks of a platter are up to 2 x faster than the innermost tracks, because more data passes under the heads in a single revolution.
This is also why the newer 7200 RPM drives nearly match the 10000 RPM Raptors in performance. They have larger platters, and with perpendicular recording, higher areal density. So even though they spin slower, they pass as much or more data under the heads every second. Plus, if you have 75 GB of data on a 150 Gb Raptor, you have already cut the performance by 25% or so 'cause the drive is 1/2 full. But a 750 Gb Barracuda or F1 will still be running on almost the outermost tracks, at near peak performance.
A better strategy, then, is to get a couple of Perpendicular drives of 500 - 750 GB. Dedicate one drive to System and Applications, and the other drive to Data. This separates the two functions between 2 sets of heads, which reduces seek times as the software switches back and forth between data, application code, and scratch disk. If you really want to gild the lily, then get a third drive to dedicate to scratch disk / swap file space.