Only buy games you want to play, right now. If you dont have plans on playing within next week just dont waste your time buying it, because chances are it will just sit on your shelf. Sure, it's fun to have a big collection on your shelf.. but for me it just wasnt worth it in the end.
I've gone thru collecting phases so many times, so if your having doubts before you even start "collecting" then don't even start unless you've got tons of money to burn. I'm sure many members will tell you the same thing. I've always felt like collecting was a empty hobby. The enjoyment I just get from spending my time spending playing a game I like is far beyond the happyness I ever got from collecting.
But that's just me