I am still using my old Matrox Millenium G400 TV 16 meg card with Vegas 4 would it make any difference to the speed of Vegas like render times and timeline scrubbing if I upgraded it to a modern one say 64 megs Nvidia.
It might help a little bit with scrubbing, but it won't make your rendering any faster. Rendering is basically Processor intensive and somewhat RAM but not nearly as much.
If you are looking at the 64 meg you may as well look at the 128 meg - Staples has both of them for the same price currently. I picked up the GeForce FX 5200 (The dual head/S-VHS output version) the other day. I can;t say that VV is any "faster" but RegGL, for example, does benifit some from it. If you do a lot of graphics work a better graphic card will help your workflow a lot. But as VV is not a graphics program you won't see that much of a boost.