I had planned to buy myself a OCZ Vertex 2 50GB SSD and use it as a boot drive and to store some apps on, but the thought crossed my mind that I might be able to get similar performance by using 2 or 3 more Samsung Spinpoint F3 500GB HDD's in RAID 0 with my current F3. Plus I would have a huge amount of storage space.
For me, this debate always comes back to the SSD boot drive + HDD programs / storage drive. Installing games / programs on a RAID drive, or SSD, will give you shorter load times, but nothing else of note (no gains while playing games).
A 60GB SSD with SandForce controller can be had for between $100 - $130USD. Add to that a 2TB HDD for about $100USD and you have a fast boot drive and lots of storage. Compare that to the cost of four Spinpoint F3 drives and you are almost at a wash. Barring redundancy reasons (hopefully you have a good backup stragegy), I would skip a RAID setup.
I know there's an array you can do where you'd have say for example 3 HDD's in RAID and then 1 HDD as a backup HDD, So if I were to go for a RAID array thats what I would do.
I can grab 3 more F3's for £100, or the SSD for £90.
Two drives in RAID0 will not be as fast. Three will be close. Would expect four drives in RAID0 to be as fast, or faster (RAID0 scales nicely). However, this is still somewhat an apples to oranges comparison... SSDs typically are faster than RAID0 when doing random reads, but slower when doing sequential writes.
It sounds like you are leaning toward the RAID setup and you will be happy with its performance. For simplicity, reduced power consumption and noise, and less hardware to fail, I would still opt for the SSD + HDD.