There are plenty of Topics created for this, search for graphic card and read what it suits you. One of them http://sketchucation.com/forums/viewtopic.php?f=15%26amp;t=21942 .
As I said, if you are just using your computer for Sketchup modeling and not doing renders with some other GPU based programs it would not be necessary to upgrade your card(unless you have money to freely do so).