Two 512GB SSDs in RAID 0 or 4 245's in RAID 0

RAID-0 is very light on the actual CPU-load. Any semi-recent CPU (even the very cheap ones, let alone the upcoming high-end Haswells) can do it with ease. Once you go to the more advanced RAID-levels, such as RAID-5 or RAID-6, CPU-performance can become a factor with larger arrays.

And even if the RAID-array used a non-trivial amount of CPU-time, it is typically still worthwhile since IO-bandwidth from storage devices is so far behind RAM-bandwidth and CPU-power that any task that involves reading or writing from/to a storage device will be limited by the speed of the storage device.

So in conclusion: I'd just go for a 4-SSD array in your case if maximizing throughput is your priority.