Can Nikon D800 use the old Nikon SB-800 flash?

I have the old Nikon SB-800 flash, and I'm intending to get the new Nikon D800. Do not want to spend money on the latest SB-910.

Most probably. To be absolutely sure, check voltages on both ends. Another way to check is to see the camera's manual on what flash models will work with it.

Any body made after the D100 can use any current (ittl) flash. So on a D800 you could use the sb-400, 600, 700, 800 and 910 as well as the ring flash setups.

You can use any iTTL flash on that camera, and the SB-800 is a very good one.