This site by Johnson Controls has a lot of good battery information.
http://www.autobatteries.com/default.aspx
The G50 uses a Group 27 battery. That covers the physical size and post configuration. Beyond that it's more an issue of what's available in your area. Some members are big fans of the Optima batteries, but I haven't had a dead battery and the desire to pay that much more for a battery strike me at the same time.
BTW, on your comment about OEM batteries - "Infiniti" batteries are just rebadged Interstate batteries the last time I checked. Here are the specs. for their Group 27 battery - which should be about as close to OEM as you would get. The physical case size should be the same... basically the difference will be weight at possibly CA and CCA.
Product ID: MTP-27(Automotive Truck) Amps: 940 Cranking Amps: 940 Cold Cranking Amps: 750 Voltage: 12 Termination: A Pro-rata Warranty: 85 Weight: 47.2 Width: 6.81 Length: 12.06 Height: 8.88
In theory the most robust battery will be the heaviest, and you want to make sure it's build date is very recent... no dusty shelf dwellers that have been sitting in the store for a year or two. I would also avoid anything that has both top posts and side terminals... you don't need the side terminals, and I had one of the unused side terminals leak on my Mom's old Maxima.
Heath
Modified by Q451990 at 2:55 PM 12/21/2008