Your PC has a 300w Power Supply Unit. Any Graphics Card you choose must require a 300w PSU or less. If it requires more you will need to upgrade your PSU.
Your PC has a PCI-E x16 1.0 slot. Most modern Graphics Cards require a PCI-E x16 2.0 slot. Your Graphics Card may still run in a PCI-E 1.0 slot but it is unlikely to function at 100% of its potential. Some users don't mind this however, and many describing the performance loss as 'negligible', it's ultimately your choice.
Here's a list of Graphics Cards and the recommended PSU wattage to power them. I like this list because it provides a high wattage estimate, ensuring the Graphics Card will receive ample power for its tasks...