my config is 2 SSD in RAID0, plus 1TB HDDs in RAID1 (plus an extra drive... because I had it laying around?).
Anywho, as someone who recently had one of my RAID1 drives fail I have to highly suggest NOT doing RAID0 for a data drive. RAID0 is for a little extra performance, or to get a larger usable pool of space when you need extra room in a single partition, but in real world applicaiton it is very risky business, especially if reusing drives that are already a few years old.
That said, HDDs have a throughput of roughly 100-130MB/s for sequential performance. However, for nonsequential performance (things like OS and program loading) HDDs have a dismal 40-60MB/s throughput. It is painfully slow!
SSDs are really not all that much faster than HDDs in real world use, and tend to hit a modest 160-200MB/s under most workloads. It is faster, but not magically faster for sequential workloads. The trick is that the HDD has a high latency due to seek time, where the SSD is under .2ms. In short, it means that for nonsequential workloads it still gets the exact same 160-200MB/s, while the traditional HDD is hitting ~1/4th the performance, plus a latency hit! That is where the magic lies. It is a huge difference, and you would be crazy to not want an SSD in your system.
For another real world example: I do a bit of video editing and from my HDDs (reading from 1 drive, writing to another to achieve max performance) I could only push my i7 to ~60% load. Put in an SSD and use that as my scratch disc? Now I have no troubble at all maxing out the load on my CPU! Also, because I OC via turbo boost (because I was too cheap to buy a K CPU... no regrets), that extra load makes the CPU go from hitting 3.6GHz, to fully maxing it out at 4.2GHz the whole time. It is a massive performance difference that I very much enjoy, and I am on cheap Agility SSDs. If you throw down some real money on a Sammy then you can get even more performance (and probably better reliability to boot), so absolutely get an SSD... it will rock your world!
And no, don't RAID0 the HDDs, not worth the risk, nor the hastle. RAID1 is fine, but RAID0 gets a bit scary and should not be done unless you keep good backups.