RAID 0 not as fast as expected

I'm experiment with RAID, looking for an inexpensive way to boost performance.  I bought 4 Seagate 7200.9 120 GB SATA
drivesand two SIIG dual-port SATA cards.  (NB: I don't plan to run RAID 0 in production, probably RAID 10, so no need
tocomment on the failure rate of RAID 0.) 

I used this raw serial-speed test:

   time sh -c "dd if=/dev/zero of=./bigfile bs=8k count=1000000 && sync"
   (unmount/remount)
   time sh -c "dd if=./bigfile of=/dev/null bs=8k count=1000000 && sync"

Which showed that the RAID 0 4-disk array was almost exactly twice as fast as each disk individually.  I expected 4X
performancefor a 4-disk RAID 0.  My suspicion is that each of these budget SATA cards is bandwidth limited; they can't
actuallyhandle two disks simultaneously, and I'd need to get four separate SATA cards to get 4X performance (or a more
expensivecard such as the Areca someone mentioned the other day). 

On the other hand, it "feels like" (using our application) the seek performance is quite a bit better, which I'd expect
givenmy hypothesis about the SIIG cards.  I don't have concrete benchmarks on seek speed. 

Thanks,
Craig