I assume you want to record at 96kbps at 44.1kHz? Doing some back-of-envelope calculations, 512mb should give you about 12 hrs recording.
To transfer your recordings out, you'll need the provided iriver manager. You can also change to the UMS firmware, which allows you to read from the player like a normal thumbdrive, without the use of the manger software, but your recording bitrate will be limited to 96kbps max.
You'll probably have to use your external mic most of the time - when using the built-in mic, there is interference (a ticking sound) in the background. There are some tips for recording in this forum, so feel free to read around
The 7xx and 8xx series are functionally the same - it's just a difference in the casing.