Graphics card improve pc performance

Yes it should, if you mean like 'PC performance' please be specific to give you good answer.

You need to give more details than that. You need budget, you need CPU model, Motherboard mode and Details on your PSU.

Apart from gaming there will be little or no performance increase except a few specific tasks that can use the graphics card. Your PC is a bit dated so it may help if you cannot play films smoothly it will help and may free up some shared RAM but nothing significant.

Not really. It only makes things that are graphics related faster, like gaming, 3d modelling, watching movies (only faster up until the point movie is smooth, it won't get better after that point). For general windows use it will do absolutely nothing. You're better off getting an SSD, that WILL boost the performance insanely.