Yes. It is a must now if you want smooth graphics. Games like Unreal Tournament, Splinter Cell, Need For Speed Underground 2 ar pretty intense on the graphics requirement.
You can always buy a new one later on and install it into the AGP port of the motherboard. Than disable the onboard graphics controller and select AGP. Then install the drivers for it.
Minimum 128 MB DDR RAM. I seen one as low as $53. But that's a cheap one. I would spend close to $100-$150 for a good one. Not the best, but will support all games.