You should continue to use the 10k disk as system disk as that's where it shines. Putting it in RAID0 with the other disks will only make it slower. For all intents, you now have three ordinary 7200rpm disks; no more 10k disk.
You realize that if any 1 of the 3 drives fail, you will lose all of your data, right?
"Speedy enough" is subjective. It will be faster than standalone drives for most tasks. You might also consider the 2 10K rpm drives in raid 0 as a boot/application disk, the other one as a standalone for data or other files, and a good backup solution.