I've got a 240w continuous / 500w Surge capacity (peak) Power invertor.

Will this be enough to run a 15" LCD and a PS2 in my car?

Here's a link: http://www.xantrex.com/web/id/191/p/1/pt/29/product.asp

 

Look on the back of the power supplies or the units themselves. You can find the power input values on the inverters. If it dosn't give you watts, it will give you voltamp(VA) or it could just give you Volts and Amps. If it gives you volts and amps, multiply the two together and that gives you watts. Now add up all the input VA and the Watts and that's how much you'll need to power those devices.

Noone can tell you if it's enough because it depends on the model as to how much power is drawn into a power consuming device.