I have 4 SATA drives running in RAID5 and i observe very poor write performance on that array.
I am runnig Debian with kernel 2.6.32-5-amd64.
Here are configuration data for my array : http://dl.dropbox.com/u/93773461/raid_cfg.txt
and here is benchamrk output : http://dl.dropbox.com/u/93773461/bench.txt
RAID 5 will have less write performance than other RAID levels because parity must be calculated and written on each write. That said, see the document on Configuring Intel® RAID Controllers for optimal performance which includes the SRCSATAWB controller.