|"Geforce or ATI? which is better??"|
They both make excellent cards. Always check the benchmark charts & read reviews before you pull the trigger on any card.
The power supply will have to be checked to make sure it can support a gaming card's power requirements. If not, it will have to be replaced. Either that or you'll have to choose a lower end card that will work with what you have.
Also, 512MB isn't enough RAM for gaming. 1GB should be considered the minimum, 2GB would be better.
I suggest you read the following:
Best Graphics Cards For The Money: Jan. '09
How Much Power Does Your Graphics Card Need?
Gaming Graphics Charts Q3/2008