The best memory cards to use for sonar recordings are the class 10 memory cards because these are the fastest there is. Size wise you need to get as large of an SDHC memory card as you can if you are going to do a lot of sonar recording (or several smaller ones). I recommend sticking with the SanDisk brand as they follow the SD Card Associations specs for SD and SDHC memory cards and should work with our units. Look at the Extreme III memory cards in at least the 4GB size if not larger. Many are keeping their sonar recordings to 30 minutes as it is easier to navigate through the video when reviewing on 3rd party software on their computers.
If you have some really old software in your 997 unit - and I’m talking older than 3.720 here, than you will have to use an SD card to update your unit as one of the updates with the 3.720 version software was the programming to allow the unit to use the SDHC type memory cards.
Can you post the url or a link to the web page that said that the SDHC memory cards would not work please?