Steve - I actually just purchased a battery from Costco about 3 weeks ago. It's the best deal out there in the market. I got the 24F and it was a perfect fit. If I recal correctly, Costo had their own name for the size. As everyone has already mentioned, it's branded as Interstate now. However, even the old Kirkland batteries were made by Johnson Controls, who also makes Interstate. I would venture to bet it's the exact same battery, just rebranded. The employee at Costco even told the that the Kirkland batteries they used to sell were delivered by an Interstate truck. Anyway, it also has a 48 month FULL replacement warranty, not pro-rated. It was about $84 plus tax, came to just over $90 out the door.
As for the dual battery setup - the only advantage to running two standard batteries would be if you draw heavy amperage at times like Ghost said (winching, etc). I have a dual setup right now, but my second battery is a small deep cycle battery that is made for discharging and recharning without damaging. This allows me to run all of my lights, inverter, etc while the Xterra is off and still have a fully charged starting battery when I need it. I didn't even bother with the Kodiak kit, I just pieced it together myself. The second battery is the smallest Optima Yellow Top made, and it's actually mounted under the front seat. I ran a 4 gauge wire up to the first battery and installed a 200 amp isolator between, which I wired to the ignition. Basically, everytime the ignition is on, the batteries are connected together in parallel (and therefore both charge off of the alternator) and everytime the ignition is off, they are seperated. All of the lighting and accessories are then connected to a fuse box that runs off of the second battery only. It's a REALLY simple setup as far as dual batteries are concerned. It was super cheap and works perfectly for what I wanted.
