How can I tell what PS3 I have?

May be...

I'm not sure what one I have xD but It isnt 20g cause it does have wi-fi and no backwards compatibility...

when you turn on your PS3 go to system information and you can see how much memory you have and then you will know which PS3 you have.

its the 40 gb if it doesn't have bc go to settings-system settings-system information and that will tell you which ps3 you have

Go to settings than system info than memory and it will tell you.

I would say 40gb, if it as wifi and cant play ps2 games!!!

yup