how to build a render farm?

For what application? The answer is yes depending on which app you are using.

how about for FCP or AE.. Is it possible?

You can for compressor not for final cut. Here is the setup for compressor. You need to set up Qmaster
http://manuals.info.apple.com/en/AppleQmaster_2_UserManual.pdf

For after effects you need to just install the render engine on all your nodes. You also need a central location for all your files that all render nodes can access. Here is a good place to start.
http://www.creativemac.com/2002/07_jul/tutorials/aerenderfarm020730.htm.

We have done some preliminary tests with compressor and had about 20 computers helping rendering. There are still some bugs that we have to work out in regards to file location. Early test results are very promising.

how about for FCP or AE.. Is it possible?

Also look at Rush ( http://seriss.com/rush/). It comes with a preconfigured script to run AE and if FCP can run via command line, then you can configure Rush to run it. We use it here to run Shake, MentalRay, and Maya over a mixed network (macs, linux, & ms).

I'll second the recommendation for RUSH. Works well with our render farm (5 Xserve Nodes) and all the rest of the workstations too. We use it for Shake and Maya/Mental Ray. There are submit scripts for AE, but I haven't tried them.

have any of you had problems with using a 10.4 farm for after effects? we're having terrible terrible write permission errors on tiger machines when we use our raid and after effects watch folder renders.

I'll second the recommendation for RUSH. Works well
with our render farm (5 Xserve Nodes) and all the
rest of the workstations too. We use it for Shake and
Maya/Mental Ray. There are submit scripts for AE, but
I haven't tried them.

I just installed AE on our render farm and will start testing in a couple days. Will post the results after the tests.