I agree with Phil's recommendations above. In the Sage 500 world, we even went so far as separate tempdb as well.
Please note, using separate partitions really doesn't accomplish the goals you are trying to achieve. The bottleneck with SQL is in disk access and therefore, you want to use separate spindles with dedicated read/write heads. The reason to separate log files and data files is the way data is written to and read from. Logs are sequential; data is not. Therefore, you will get better performance if your log files are on a set of drives where the read/write heads can keep from bouncing around. For reports and queries, tempdb can see heavy usage so separating it from data access optimizes the system to maximize the bottle-necked resources; the read/write heads on the spindles.
I hope this helps.