I'm not surprised to see that kind of load with that processor and that setup. Software RAID tends to chew up processor cycles. As do ATA drives (even in UDMA mode). I'm really not sure that you can make it a whole lot better with the current hardware.
Ideally, you would run a busmastering SCSI RAID card... Sometimes you can run across those cheap at swap meets. Of course, if money grew on trees, a nice new Opteron or D-840 would solve the problem too