Your 300W PSU is a tier 4 which is not recommend for use and If you upgrade your graphic card then you need a better PSU.
With the i3 CPU you could get a 6750 or a 6850 with a 450w PSU. You also might think about better ram or just faster like a 1600mhz.
Here is a link to PSU's from the best to the worst. If I get time I will look for links to newegg for the psu and gpu. Good luckhttp://www.eggxpert.com/forums/thread/323050.aspx
edit Don't get a GTX 550Ti it is the worst of the Nvidia 5 series cost to much for what it can give.