Uprade help needed for Emachine ET 1331G 03W

I currently have E-machine ET 1331G 03W. 300w PSU and integrated Nvidia GeForce 6150 SE, currently using AMD Athlon II 235e 2.70 GHz. was thinking of upgrading to a EVGA 550 TI. Only has PCI Express slot . Currently running 6 GB ram. limited space for video card. need info for PSU to use.
Any help would be great.