I'm very new and still demo'ing open-e but I stumbled upon this post and wanted to throw my S/W RAID5 build performance numbers into the ring.

SuperMicro Superserver 6015V-MT
1x Xeon 5120 Woodcrest (dual-core 1.8)
2GB DDR2 667mhz
Onboard SATA controller
4x 1TB Western Digital SATA

We're getting sustained resync (build) rates of over 80,000K/sec, building out that 4TB array in a little less than 3 hours.