The standard IP-35 DOES have built-in RAID capability, as well as Firewire, but not eSATA. The IP-35E does not have these; it is just basic functionality. The IP-35 Pro has all these, as well as a better heatpipe cooling system, and some other nice bells and whistles, such as a diagnostic LED display. Seeing as you use an external hard drive, I'm willing to bet you'll need the eSATA support, so the IP-35 Pro is what you'll need. At least it has built in 10/100/1000 Ethernet and onboard sound (though if you plan on gaming or doing any audio work, or are an audiophile, I'd recommend a add-on PCI sound card like the Creative X-Fi XtremeGamer).
The only reason I know this is I just ran a thread asking about the Abit IP-35/IP-35 Pro; I'll be buying one or the other next week.