I've been into classic game collecting since the mid 90's (back when the real Atari was actually still around). Up until last year I had a massive gaming collection that spanned multiple systems from the Atari 2600 to Neo Geo (many were boxed as well). For the longest time, anything classic would sell. Loose 2600 games, common 5200 games, Vic-20 cartridges, Colecovision stuff, anything as long as it was pre-1985ish. People were reliving their childhood, only this time they had access to a much bigger allowance and they wanted everything they were denied back when they were kids. Then after awhile I started noticing that no one wanted the loose stuff anymore, people were now paying big bucks for manuals and boxes (originally people wouldn't give you much if anything extra for the box). The reason behind this was that all the big time collectors had the loose games, now they needed something new to collect so they went for the extras (boxes, manuals, catalogs, etc.). Loose games would sit there on gaming convention tables gathering dust other than a handful of very rare titles. Now we're getting to the point where the big time collectors have all the common and uncommon stuff they need, boxed and otherwise, so it's only the rare and extremely rare stuff that's selling. Those will always be worth money because there aren't enough of them for every one to have one. So everyone that was hording common and loose classic gaming stuff like it was gold are discovering that their Pac-Man cart is worth exactly 10 cents and not the $10 they were lead to believe. Unfortunately it would appear that many brick and mortar gaming shops still haven't gotten this memo.
Another thing to consider is the age of the collector. Back when I got into the hobby (mid 90's) Pre-Nintendo stuff was all the rage because that's what the current collectors grew up with. We were all 20 to 30 somethings who grew up with a 2600 joystick firmly affixed to our hand and that's what we wanted to collect for. However about 7 or 8 years ago I started to notice that the classic stuff I grew up with wasn't selling as much as it used to, and it was NES stuff that was starting to go for big money. I found this odd because up until that I point I was grabbing NES games out of bins at flea markets for $2 each, and suddenly even the common games were going for six or eight bucks, while boxed games were going for $80-$100 or more depending on the title. Then it occurred to me that the kids who grew up with the NES were now old enough and wealthy enough to start buying all the games that they missed out on as a kid. So the valuable and collectible games had shifted from Atari era stuff to NES era stuff. That doesn't mean that the Atari stuff was worthless now, but only the rarer stuff kept its value, the rest started to slip. Now we're starting to see SNES and Genesis stuff rise value (the NES stuff hasn't started to fall off yet, but its coming) and eventually we'll see the Saturn and PSX stuff skyrocket as well (although the rarer stuff already has).
So my point is, yes classic gaming can be a good investment, but only for a short time. However unless you're constantly selling off and buying at the right time (before the next trend hits) you're eventually going to lose money or at best break even. The days of mega cheap games that are going to rise in value are over, because people are already looking for what's going to become collectible in the future even with the current stuff (sort of like comic books). That's why we always say not to get into classic game collecting for the money, because there really isn't any. Get into classic gaming because you love the games.
I got lucky because I bought the bulk of my collection when people weren't thinking of what it would be worth in the future, we were thinking of the here and now. When I decided to sell off my collection due to an upcoming move to a smaller house, I actually made a good deal of money on it. However that's because I bought it back before the collecting boom happened, if I tried to recreate my collection now (or even ten years ago) and then sell it I would have lost a fortune.
You can see pictures of what my collection used to look like here:
http://atariage.com/forums/topic/146921-tempests-game-room/ and what it looks like now that I've slimmed it down (it looks bigger, but it's not, I just concentrated everything into one room):
http://atariage.com/forums/topic/210908-tempests-new-new-game-room/