The problem with RAID0 for speed increase is that the main thing you need speed wise for most applications is seek time, and RAID0 doesn't do anything for it.
RAID0 gives you a huge boost to sequential transfer rates (theoretically doubles it)...but unless you are reading sequentially from one gigantic file that is totally contiguous (defragged, to use the windows term) that really doesn't gain you much.
Even reading large files under normal circumstances you have to jump around the disk because the files blocks won't be contiguous.
So it does depend on what you are using the disks for, there are certain very specialized tasks that RAID0's speed increase can be significant for. But usually STR is a very small part of the overall performance picture, so even though RAID0 increases it hugely the overall result it not that significant. Unfortunately STR is also one of the easiest things to measure the performance impact of, so synthetic benchmarks are often much more dependant on STR than real world use.
If you want hard drive performance increases (which you should, it's often a performance bottleneck), you want to find yourself the drive with the lowest average seek time. Don't worry about STR.
RPM actually is an important measure, it contributes to both lowering rotational latency (which is a component of total seek time) and increasing STR, but it's not the only factor. Big buffers are great too, since if something is the buffer it can just be sent out, no need to move the read head at all.
RAID5 in theory should be able to offer the similar read performance improvements to RAID0, but also includes some redundancy. It's a pretty good bag if you can afford those drives. I think 3 drive is minimum for RAID5 and you'll get total capacity of the array equal to 2 drives (ie, 3 200GB drives in RAID5 gives you 400GB total space), with actual redundancy, you need 2 of the 3 drives to fail to lose your data.
I haven't used RAID5, so in practice I'm not sure what it's performance is like. But it seems like a pretty good deal to me. RAID0 is a pretty risky game for probably minimal performance improvments.
