The 6.5 or 7 is the width of the wheel, don't worry too much about this. The narrowest tyre you want is a 195 but I think the 205 is better. You should also be able to get a 215 tyre on but may get some nasty chafing :O
They are the wrong size for the 155 though.
Should be 205/50/15 or 195/55/15.
Good point Marlon....only let down by the fact that the difference between 195/50 and 205/50 is -1.71% or -10mm :O and the difference between 205/50 and 195/55 is exactly the same error but positive +1.71% or +10mm....wierd huh!
So the choice is 205/50's or plus/minus 10mm :O..there is no tyre size nearer for 15's.