RAID 0 and RAID 5 can improve transfer rates, but not access times.
RAID won't give you much of a speed-up for general-purpose activity such as booting and starting applications because they're a lot more dependent on good access times.
It will help with copying or reading/writing of large files, though.
Note that the WRITE performance of RAID-5 is VERY POOR, so it shouldn't be used for write-intensive files.
You should also avoid using RAID-5 for very large volumes (more than, say 2TB or so) because most disks don't have a high enough read reliability rate to provide a good chance of recovery after a drive failure. If you need a very large redundant array then RAID-6 or RAID-1 is a safer choice.
I might be a week late in this process but hold on more drives better up til a point and wait 4 is all I've tested. 5 or 6 might be better..
I have four F3 1TB and in RAID0 using the first 400-800GB size in volume perform near 500MB/s 12ms seek times. Also using on board from Intel ICH10R RAID5 scored better results than in RAID 10 by close to 100MB/s using HD Tune with 4 drives. These drives appear to be what the tomshardware RAID scaling charts are based. After having 4 of them in RAID 0, 1, 10, and 5 my more ideal question would be does the Spinpoint perform better using 4, 5, versus 6 in RAID 0, 1, 10, 5 because I have found them to more than meet my expectations in adding more drives is better at least up to 4.
My Answer: 4 drives RAID0 for best performance. I have benchmarks to prove it. Doesn't matter how you arrange them but if RAID10 or RAID5 was on the table I would be undecided because RAID5 is performing better using 4 against 4x 01 config and I need to experience the "needs initialized"