Since there will be over 1 million units for the North American launch, you should be able to grab one as long as you get there a little early. I'd recommend you get there later in the afternoon around 3pm and check to see if a line has formed. Then it's just a matter of how early you want to hop in line and get ready to wait. Don't forget, if you miss the Walmart midnight launch, you can always hit Best Buy the next morning as they generally get the largest allotment of systems on launch day.
As for the Genesis and TG-16 games available at launch, here they are:
TG-16: Bonk's Adventure New Adventure Island R-Type
Genesis Altered Beast Ristar Sonic the Hedgehog Columns Toe Jam & Earl Golden Axe Ecco the Dolphin
Commodore 64 Jumpman Jr. Pitstop Summer Games Summer Games II Winter Games Impossible Mission Impossible Mission II
NES: Legend of Zelda Super Mario Bros. Donkey Kong Mario Bros.
Super NES: Super Mario World Legend of Zelda: Link to the Past