Advent DT2412 This one I am assuming, it is not really a gaming PC to be honest. It has a rubbish graphics card and you can do a lot better for £800 elsewhere.
I am not sure what you mean by your second question. Are you asking about upgrading your sons current PC with the 660Ti or the new one either way it it will depend on the power supply each PC has as mogern graphics cards can be greedy for power. Also you would need to provide more details of yhe old PC as to whether upgrading the graphics card would be a good idea.