The X25-M G2's do seem rather good, I'm still concerned about wear leveling in general on SSD's (have burned up some SLC based drives already, though my access patterns are probably /much/ higher than the norm).

Random IOPS are a derivation of service time of the drive. For traditional drives that's pretty much your rotation rate, for SSD's it would be the design & speed of the nand chips (number of channels, how many cells per block for the erase cycle, et al). Streaming is mainly based on bit density. RAID itself doesn't really provide improvements unless your queue depth is higher than 1 (and usually much higher than that to really show the benefits) also for streaming you need to have request sizes that span multiple stripes. For writes likewise you need to really operate in full stripe widths to avoid the penalties of parity based raids.

Parity raids are generally chosen for storage efficiency over performance first, then for availability (raid6 > raid 1 > raid 10> raid5) then for block level integrity checking (not to be confused with data integrity as a raid doesn't care about your data really). To over come the write performance hits and to minimize rebuild time you create multi-level raids (raids that are striped) the simplest is raid 10 (1+0) but that also extents to any raid type (5+0 for example that we talked about above).

All things being equal, the more drives the better. Your comment above about system performance dropping during backups is very appropriate here. file based backups (opposed to block or file system level) suck up a lot of overhead. This is a VERY pertinent issue in enterprises and why things like synthetic backups and similar are getting to be very popular as systems can't perform the lookups fast enough to feed the backup systems. (for say a ntfs directory with the normal small file types (.dll's et al) it takes a LOT of drives to avoid a tape drive from shoe-shining (about 80-100 7.2Krpm drives or 40-50 15K drives for ~100-120MiB/s)). IOPS are a finite resource which you have guard in your subsystem design.