You should get a high quality 350W PSU, or a cheaper brand 400 or 450W PSU to make sure you have enough power for the GPU. Using PSU calculators online using standard settings (several USB devices, a DVD drive etc) you could be using around 300W and you never want to be using the max a PSU can put out.. Get a bigger PSU and you'll be fine.
Also.. the GT 240 is a brill little card and surprises most with how powerful it is. It can play the latest games on med/high/highest settings at very good resolutions (1080p for example).