For games usage the best memory is that which comes with the card - i.e. is not a separate plug in module.
Look for at least 256 Mbytes graphics cards for good performance.
Take a look at
review of graphics cards and this
group test Also remember there are two types of graphics cards for the PC - either PCI or AGP. The connections are different and you need to get the right one! Check with your PC's handbook if you're not sure.
HTH.