Nothing will happen to your 300D. I've stuck my 350D through a few times already after only having it for a couple of weeks and it's fine.
I'm assuming the 300V is a film camera? Same thing - nothing will happen to it. But if you are using high speed film (400 or higher) - don't leave the film in the camera, coz the checked baggage xray machines are apparently stronger. Take it through in your hand luggage and if it's 800 or more get them to handcheck it. Otherwise jsut chuck it through with your handluggage.
Just think back to last time you broke your arm or something (hopefully never). You know how you get an xray of your bones. The bones show up because the x-rays just bounce right off them (like light) and the dark bits are where the xrays just vanish off into the distance. So your camera is just like the bones - it just bounces right off the metal/plastic.