well..it is depend how many Ram Slots do you have and how many Ram slots did you used for 240mb. if you still have 2 Ram slots left, you can just add one 512 mb or 2's 256 mb rams.
For my sugguestion.. i think you shouldn't just upgrade your Memory, you should upgrade your CPU, motherboard too. PIII right now cannot do much on nowaday application. For example, if you are using MS Office 2007, you computer will be very slow.