1) Remove all mods that change weather or the lighting.
2) The texture models you download from New Vegas Nexus also take a bite out of your processing power.
3) Don't hit Ultra settings from the get-go, set it to High and lower and raise things as you deem fit.
4) Update your video drivers if necessary or go into the NVidia control panel and make sure its set to allow applications to control the GPU directly.
5) Lower your anti-aliasing settings to modest levels or lower water settings to medium.
6) The game employs use of your hard-drive to cache & set draw distances which could stutter gameplay if your HD has slow RPM.
For reference, I'm up on the latest update with about 2 community mods (Veronica custom face-mod, wide-screen text fix) and I'm playing at High settings on my 8800GTS (640mb of VRAM).