No. It's the memory bus width. That and the video card's memory type and the speed of that memory determine how fast the GPU communicates with the memory.
Before you buy anything, could you post your Presario's model number? Apparently, it's not as old as I thought, and you might be able to upgrade it to something a lot more powerful (able to play modern games) for not a lot of money. AMD CPUs are cheap.
"Mind your wants, 'cause someone wants your mind."