OK!

As you have the AGP card slot, that is what you want. It connects at about twice the speed of PCI and five times the speed of ISA (don't ask............I will tell you tales of dinosaurs and dragons)

ISA slots lay eggs!.......................true!.........................

I do not know your pricing structure, other than that it is quite different in India from here; neither do I know the games you like to play.

However, as a gross generalisation games fall into two sorts:

1. Memory intensive
2. Graphics intensive

I will make a guess that your games are graphics intensive, and that framerate is important?

So, you need to look at the video card in two ways:

1. Total memory (RAM) on the card..........64/128/256
2. Processing speed.........the card has its own processor, which has a speed.

It is thus possible that a 128 is better than a 256 for what you want to play.

From the rest of your specifications it is not worth getting anything better than a 256, and I would suggest a 128 (if there is a significant price difference) and putting the money saved towards a new machine with faster BUSes, memory, and a better video card.

It is all a matter of balancing your kit