It depends on the video card.
make sure the 256 mbs of ram is at least as fast as your old video card’s ram
Also make sure it has quartz extreme built in. This is one of the core reason why OS X has such great graphics and effects. with out quartz extreme you’ll notice the computer’s OS graphics running slower and some times not running at all. You need quartz extreme for good video and DVD playback as well.
Also make sure the GPU of the video card is fast enough as well. From my experience, the best looking video card may not be the best over all. Memory isn’t everything, the PS3 has 256 mbs of video memory and it can do tremendous graphics.