How much it will make difference totally depends upon the fact ...like what actually you run...with windows 98 and bit of office tools its really not worth anything....But step into windows XP and gaming zone....and man are you in new world ?
256 mb ram will make a lot of difference ....thats where the eye candy of windows xp and most of the games live....Hmmmmm if you are talking Graphics animation...you are better off with 512 mb Ram or more pal...
..any way things finally depend on what you run ...!!!!!!!!! or intend to !!!!