Hi Jody,
This sounds great.
The cartridge port works very similarly as for a C64, and most C64 cartridges will work. For expansion memory, the MEGA65 will have 8MB of ~10MHz expansion RAM in the final version, but in the meantime, there is 384KB of main memory, that the VIC-IV and CPU can both access, as well as being able to access the SD card at between 700KB/sec and 1.3MB/sec already.
The CPU is an enhanced 6502, with memory banking commands, and also some instructions for directly accessing any byte of memory without having to change the memory bank layout, which helps keep code small and fast.
Basically if you have questions, or need information, then poke us, and we will do our best to support you, as we are also excited about the prospect of MEGA65 games.
Paul.