If you think that there are a lot of IP35 versions you haven't seen Asus P5Ks or Gigabyte P35s ...
IP35-E uses ICH9 so 4 SATA & no RAID
IP35 is the same but with ICH9R (6 SATA & RAID)+ firewire+ different heatsinks
IP35 Pro adds uGuru, 2nd physical x16 PCI-E slot, 2nd gigabit LAN, 2x eSATA, all caps are solid, onboard power & reset switches+ external clearCMOS, slightly richer BIOS, different again heatsinks