Yes, if you have FFDShow installed you don't need almost anything else.. but if your coputer configuration is pretty recent (CPU > 700MHz etc...) it's better to install the proper codecs for relative filetypes (DivX, MPEG2, XviD and Div3 should be enough)..
Take a look at adicoto's list of codecs which link you can find in BSPeter's "READ THIS BEFORE POSTING" post (aka Help2Help)
And which version of FFDShow do you have installed?