|The GT 240 is a good entry level gaming card with very low power requirements. Chances are good you didn't need to upgrade the power supply. That being said.....|
"The new power source is 500W so having enough power shouldn't be the issue"
Wattage is NOT the primary issue to consider when choosing a PSU, amperage is. Plus the PSU should be made by a company with a good reputation for quality & reliability. Price is a pretty fair indicator of quality, so is weight. A cheap PSU is usually made from low cost/lightweight components (lightweight heatsinks, cheap capacitors, light gauge wiring, etc)...that's what makes them cheap. The makers of these cheap units tend to exaggerate their wattage & amperage specs too.
What I'm getting at is you can't make a blanket statement that the PSU isn't the problem because it's a 500W'er. It's completely possible that you downgraded if you didn't get a decent unit. How about posting the make/model, amperage specs & what you paid for it?