The P4C800 DLX uses an ICH5 and 3Com GbE NIC.
The P4C800-E DLX uses the ICH5R with RAID and the Intel CSA GbE NIC.
As a result, the P4C800-E DLX allows for two RAID arrays, from both the Promise Controller and the ICH5R, and has less PCI traffic as the NIC is using the CSA connection.