I am not sure what you are trying to achieve. I shot something recently in 24P then selected TINT in one of the video filters section which basically desaturates the clip to a black and white look. It looked good to me.
I'd throw a slight blur too. I don't have the link, but I just read a great article in Corporate Video on a project to simulate old footage. But they were looking for more than just b/w, but an age look too. Alot was actually done in-camera rather than post, but looked pretty cool. If you know anything about After Effects, you may want to try the blur filters. The idea is to simulate a narrow DOF. The idea is to only have in focus the 'focal point'. I don't think FC allows this type of variable control.
I've been trying to avoid laying out the dough and climbing another learning curve, but it's increasingly looking like I'm going to have to get After Effects. It never ends...
Before going to after effects take a look at all your filters available in FCP. Once you've removed the saturation (color) they try out the levels or contrast controls if you want it snappier and more rugged. FCP also supports QT filters, some of which allow you to put scratches, hair, or sepia tones to your footage. You should be able to add noise with a slight blur if you want to simulate grain. It's worth experiementing for a short time to see what you can get.
In the case of Jarmusch you should apply more grain. And a lot of contrast. I think down by the law was filmed at 16mm so the upreez to 35 gives that grain. Also was filmed by the same dp (can´t get the name right now) of other movies like "the American friend" who had a lot of grain too.
For vertov look try with grain, hair scratches... But he never had shot equals. Better don´t try to imitate what it is inimitable. At the end we create monsters.
for $99 you can use film effects by nattress.com. I probably sound like i'm paid by natt, but no, i just love his filters and am impressed that he has not jacked his price even though many also LOVE his filters.
also..i believe there is a filter in QuickTime pro that will add dust and scratches type stuff to it.
otherwise, you need to crush the blacks a little ( in addition to the other tips others gave..)