RAID 5 reading performance

I have a Dell PowerEdge 2400 with a PERC 2/Si RAID controller onboard
(Adaptec 7890 + Intel i960@100mhz + 64 mb RAM). I want RAID 5, as RAID 0
with 6 disks is a sure way to loose data and RAID 1 wastes too much space.
My problem is that reading performance isn't very good. I get roughly 32
mb/s from the RAID which consists of 6 18 gb 10K RPM SCA disks of different
brands and models. The same disks get around 52 mb/s on my other
controller, a Adaptec 29160.

The two controllers are placed on two different PCI busses. The first one
holds all onboard devices, including the PERC and a ATI Rage graphics
adapter. The server only runs in textmode, so the Rage shouldn't be able to
starve the PCI bus. The second PCI bus has 5 64 bit slots and holds 2
ethernet adapters, one gigabit and one with 4 100 mbit ports, the 29160,
sound card and a USB 2.0 card. Chunksize on the PERC is set to 32 kb and
the OS'es i've tested with was Linux 2.6.20 and FreeBSD 6.2. How can i
improve performance?

--
Lasse Jensen [fafler at g mail dot com]
Linux, the choice of a GNU generation.