When designing under css, what techniques do you guys use to test across different browsers?

i feel like im doing things the idiot way. I never went to school for web development or designing. So everything was all self taught from reading or asking nicely. However, here is how i do it (please show me if there is a better way), i would first design for firefox (since safari, chrome, and opera seem to act all the same way as far as margin and padding rules go), then last would be IE. But i guess the main issue would be the different versions of internet explorer. Currently i only have ie 7 installed and i know for a fact that things wont go right in ie 6. So what do you guys do in these type of situation?