I have a late 2008 13 inch uni-body Apple Macbook 2G core two duo, 4GB of ram, with an Nvidia Geforce 9400M graphics card. I recently bough a 27 inch Apple Cinema display and when I plug in my Mac it isn't preforming well. Its laggy. 

Will upgrading my hard drive to a Solid State Drive improve the performance? Is there any other ways to fix this issue? 

Thanks. 

Edit: updated the type of graphics card