Some RAID levels that implement striping can improve performance.  Choosing a RAID level usually involves a trade-off between performance and reliability though.

Putting the log file on a separate drive helps because the log file requires sequential access, where the data file requires random access.  An optimum RAID (performance-wise) solution might be to put your log file on a RAID 1+0 separate from the data files.  You get the performance boost of separating the log files out which reduces head-thrashing, plus a boost from striping of the RAID 1+0 and of course RAID mirroring.  Of course RAIDing out everything costs $$$ which is usually the final limiting factor