For that low of a resolution, and an i3, you'll be OK with pretty much the lowest gaming card that is worth getting, Radeon 6770, unless you want to hold on to it for a few years and move it into a faster system. Then you can look at something higher end, click on the link in my signature for some options. A 480 watt power supply should be able to handle cards into the mid range for gaming, unless it's a cheap brand power supply, many of those put almost half of what they say the rating is.