The "A" in the KT7A signifies that the newer board has a 133MHz(x2) FSB speed, over the non "A"'s 100MHz(x2). IDE RAID is most definately NOT faster that SCSI RAID, and this should be obvious, but it IS hella lot faster than non-RAID IDE. But you do need more than one hard drive if you want to take advantage of the RAID. If you dont want to spend more money on hard drives I'd go with the reg KT7A. If you DO have multiple drives I'd get the KT7-RAID.
BTW: I have the KT7A-RAID and it kicks ass