tbh dont bother with raid nowdays i used to but not anymore its more pain than its worth i think.
especially if your an addicted upgrader
Don;t know where you're getting the "pain" from. Been running raid 0 for year on my OS disk for years without trouble.
You said you have:
500gb
120gb
250gb x 2
The biggest performance increase that I've seen was when going from 1 to 2 disks in Raid 0. Beyond that, the increases were not as noticeable - probably measurable, but certainly not significant enough for you to feel the difference.
I'd be tempted to ignore the 120GB drive and use that as a backup, and use the 2x250gb in Raid 0 for the OS and 500gb for local storage/backup. You could include the 500Gb drive in the array, but as I already said, I don't think you'll be able to discern the benefit.
If you also consider that usually the smaller drives are older, and so the older drives usually have a higher latency, negativing part off the benefit "upgrading" the array
If you just want speed, then 4 drives in Raid 0 or Raid 5 would give you a capacity of 120gb, wasting a total of 130+130+380=640gb, but as said previously I can;t be sure how much quicker that will actually really be.