I have been doing HEAVY garage sales for over a year now.
I have had some KILLER scores over the past year.
That being said, here's what I have NEVER found in the wild:
Atari anything. I found ONE Atari 7800 system once, and since then I've not seen so much as a loose cart or controller.
For that matter, I have never found anything Intellivision or Colecovision related at all. Not a thing. Not once.
I have never found any NES or SNES games at a garage sale. I have seen one person with an NES and Mike Tyson's Punchout, but they wanted $80 firm for it. I have found one NES front loader system (at the same sale I got the 7800)
I have found a TI-994a once. Other than that I have never found a single classic computer system in any way, shape, or form.
Here's what I do find a lot of:
Gameboy systems and games, sometimes DS, even found a PSP and some games for it from time to time. Handheld game system and games are fairly common-ish. You'll likely find DS, GBA, and Gameboy stuff. Sometimes even Sega Game Gear.
Speaking of Sega: I still find a pretty regular amount of Sega Genesis stuff for whatever reason. Still find systems sometimes, and a loose game here and there. Not sure why I find that more than Nintendo, but that is definitely the case.
You'll find LOTS of Wii, XBox, Xbox360, PS2, and PS3 games. Lots of them. By far the most common. Not a bad thing for me, but I rarely keep anything out of those finds.
I also tend to find lots of Guitar Hero controllers, Rock Band controllers, and odd driving controllers etc.
You'll find lots of terrible PC games, but keep an eye out for the occasional big box PC games. Some of those fetch a nice price.
All in all, I enjoy it. I get VERY little for my own collecting out of them, but I get lots of flea market/eBay fodder, and some nice trade goods. I also find lots of old board games, sometimes electronic games, etc. The hunt is fun.
Your area might be totally different, and I find that some areas aren't as good as others. For example, very affluent neighborhoods are usually terrible. Multi-family garage sales are awesome. Church garage sales are very random, and I find very strange things there.
Make a day of it, hit the thrift stores, etc. And let us know what you find!