The 82801AA ICH is the ICH used in the i815. Its main features are controllers for: 133MBps PCI, ATA/66, AC'97 Link (for audio and modem applications both integrated and through an AMR slot), ACPI (power management), Alert on LAN, floppy, serial, parallel, IR and two USB ports, and an SMBus interface (for communicating with a battery). This is a feature set you should be well familiar with, as it is the same feature set you would see on any i820 motherboard.
The 82801BA ICH is the ICH used in the i815E. For a list of features, take them all from the original ICH and add controllers for: ATA/100, an integrated 10/100/HPNA LAN connect interface, dual master USB (adds a second 12Mbps USB controller), and a Communication and Networking Riser slot (CNR).
The ICH2 supports four ATA/100 connectors with support for two master drives and two slave drives. ATA/100 is the newest form of ATA to come along and ICH2 uses it to read from drives at up to 100MBps and write at up to 89MBps. ICH2 still supports ATA/66 and ATA/33 as well as BMIDE and PIO modes, so your old drives should work just fine with it. As far as performance is concerned, ATA/100 should increase performance by about 10% over ATA/66 when using a single ATA/100 drive. There is no ATA drive available today that uses all the bandwidth of ATA/33, let alone ATA/100. The performance boost is mainly from the increased bandwidth speeding up cache access over the ATA/100 bus.