You're going to gain almost nothing with RAID-0, unless you have a second array of equal or greater speed. Also, RAID-0 with different drives is rarely a good idea.
The best use of your money will be to buy the second Raptor of any size, and use the 2 drives separately, one for OS and applications, and the second as a scratch disc where you place your temp files or anything else that will benefit from the speed gained by having 2 Raptors talking to each other. If your old Raptor is a SATA1 model, then you will see a boost in speed with the newer 16MB SATA2 models, so the newer one would be the choice for OS and applications.
Once again, the speed gained from RAID-0 is in sustained transfers only, so unless you have 2 arrays working off each other, you won't see that speed.