I asume you have a pcix MoBo? Dave is right, you are not going to see real benefit with a video card swap. Most decent cards reqiure +500watt power supplies, which you may or may not have. After 3 or 4 years an editing pc really becomes fustrating to work with for all sorts of reasons.
If you must, I'd see if the MoBo would support a hotter CPU, say an AMD 6000. They are pretty cheap now, and that would greatly speed encode and CPU Fx.