keep in mind that a video card doesn't just fall to pieces when you're using more VRAM than it has. it will begin to swap with your system RAM, which is slower but still works. i think the general rule of thumb is that framerate won't really plummet on a 512 MB video card until you start using 600 MB or more. beyond that point, your framerate will begin to drop dramatically. however, it's still possible that having each video card render every other frame will still keep you rendering faster than a single GTX. if you have a GX2 and each GPU is only rendering 30 fps, you're still getting a total of 60 fps assuming you're running in AFR (Alternate Frame Rendering) mode.
Bookmarks