There is a computer game speed issue I have, can you help?

Just bought a new graphics card MSI NX8800GT (NVIDIA GeForce 8800 GT) for my computer and I had problems playing some new games I'd bought. Now here's the thing - I have a 2.80 gigahertz Intel Pentium D, 16 kilobyte primary memory cache, 1024 kilobyte secondary memory cache, 2048 Megabytes Installed Memory, the Main Circuit Board is a ASUSTeK Computer INC. P5RD2-VM Rev 1.xx, Bus Clock: 200 megahertz, BIOS: American Megatrends Inc..... and although the graphics are improved the game play is jaggered and slow... I'm not great with computers so can someone tell me what i'm doing wrong to get games like Crysis and in's Creed to run smoothly!? What is it I acutually need to do? Any help would be appreciated. My thanks in advance.