Re: SD card maximum size?
I've used upto 4G cards in my 798. I think I've seen discussion that it will work upto 16G, but call HB tech support to be sure. The 898 was new last year, and suspect 4G was always supported. Unless you are doing ALLOT of recordings, you would not need anything that large. For recordings, you want the fastest chips. I am currently using a 2G Extreme III, but believe there is a slightly newer (speed wise) chip you can get now.
Update: You want Extreme III professional with 30mb/sec transfer rates. A 16G is around $130 though.
BTW: Check your HB account for a new S/W release. Version 5.210 is available for most units, and coming soon for others.
Update: For your unit it is Version 5.320 and here are some of the new features:
http://www.humminbird.com/FAQ.aspx?Contentid=3024