I go on collecting sprees for random systems every now and then but it would be way too costly to try to complete a collection for a certain system. Half of the games suck and I'd never want to play them anyways.
I have maybe 90-100 NES games, 30 SNES games, 70-80 N64 games, 30 cube games, 10 Wii games, amongst other systems.
If I find myself thinking hey...I really want to play ______ game then I'll track down a copy. Or if I randomly see a game or two on craigslist that I've been thinking about or have thought about finding I'll pick it up. My collection is slowly growing in no particular order but this way I make sure I enjoy whatever games I pick up. What's the point of having a shit ton of games that I'll never play? I know one day my kids or another collector will look at the games I have and say that is one good selection of games. I buy with no intent to get rid of them one day.