If i play online ps3 games how much downloading am i doing

It depends how often you play, but it wouldn't be anymore than using a PC game online, with "Teamspeak" or "Ventrillo".
For me on my PC (with team speak) 1GB should last for a week and a half, playing for 5 hours everyday. That is a rough estimate. It SHOULDNT be much different from a PS3, seeing as its the same (if not less.
If you don't talk to people online wile playing, that will save you MB's of internet use, as it wont stream constant wave files at you (which are large).
Hope this helps. Try and find some info out on PC gaming, and it should be the same :)