I use on-board video for mine, and 2 gigs of ram. Video cards are used for displaying video for things like hardware stretching or scanlines on a high res monitor. Basically imagine the card rendering a pair of polys to form a square, and putting the game video on them as a texture. I perfer directdraw -hw stretching because I like my arcade games looking jaggy (feels more legit). If you have an arcade monitor or some other low res device, you won't have any problems with all of the oldies, scanlines, whatever whichever. But, if you want stuff like that on a high res monitor (in directdraw) it will destroy your performance.
You want a solid boost? Try going 64 bit if possible. If your P4 has Hyperthreading, it should have 64bit capabilities. (I don't think regular old P4s have it, as they were made before that feature was being implemented for the general public)