8-cell batteries work wonders for getting more power, but your mileage will vary in total time. I run transcoded AVI files at a screen resolution that's half of the mini 9's (just in case I want to look at the file on my old Axim 30 instead), and they look good enough for in-seat viewing on an airliner at 700 MB each.
I run them off a 16 MB stick, and my average is close to 5.5 hours at full screen illumination with my 8-cell bundle. Of course, you have to deal with the flashing LEDs and the power-down-before-recharge that's the bane of non-Dell batteries, but that's minor.
One thing to remember, though, is that you're going to have the swing of the screen impaired with the larger battery -- i.e., the screen isn't going to go back as far as it does with a standard 4-cell. You may need to prop up the front end of the keyboard base to get a better angle for viewing. Again, it's a trade-off.