With the older drivers, my GPU 0 temp used to peak around 72C when working hard on FNV, with the other two cards not getting much hotter than their normal 52C-ish idle temp. With the 260.99 drivers GPU 0 peaked at 69C and the other two cards peaked at 65C and 62C after a one-hour continuous FNV session.
My completely unscientific, wild-ass guess on this is that FNV is now finally using all three cards to render and doesn't have to skip frames to keep up with movement.
However, I still have to use the d3dx9.dll, otherwise the movement stuttering and NPC slowdown is back to the usual abysmal performance if it is removed. So, another SWAG on my part is that this game engine still hates DX10/11.
Rig: i7:920 stock clock; 3x SLI GTX260; 6GB RAM; Win 7 64-bit; Samsung 27" monitor
Disclaimer: this works for me to improve my FNV experience. I share it in hopes that it may help someone else. If it does, great, if it does not, then you're no worse off than you were before so there's no reason to lash out at me. Thanks!