Just wondering. I had to run Fallout 3 on lowest settings, otherwise it ran perfectly fine.

Pretty much. They both run on the same engine and graphically they are practically identical (in terms of resolutions and effects). The only place in the game which may give you slow downs is the Strip but if you keep your settings on low then it probably won't matter. If you do play New Vegas I recommend using the Stutter Remover and New Vegas 4GB Enabler as they will boost your performance significantly.