A 300 Watt psu is not enough, especially if you plan on upgrading to a decently powerful dedicated graphics card. As for the card itself, get the best card you can afford. Historically, AMD provides the best bang for the buck when it comes to budget oriented builds. I'd recommend getting a 500 Watt psu for the build. It will give you enough room for future upgrades and can run any graphics card you purchase. What is your price range for a video card?
**edit** also a side not when shopping for a psu, make sure you have enough power connectors for your GPU. Your current psu has only a single 6 pin connector while most graphics cards have 2.