my system

Motherboard: intel dg31pr
GPU: nvidia geforce 8400 gs 512mb
CPU: intel core 2 quad q8300 2.5 ghz 4mb cache
DVD drive: LG GH22 22x (dual layer)
RAM: 4 gb
Harddrive: 160 gb

can I add an nvidia geforce 8800 to this system

Depends on what slots you have. You can't add a new GPU to a motherboard. But you can insert a new graphics card into a slot. Yopu don't say whether you have an AGP or PCI-E slot available. If you do, then get a card for the appropriate slot type.