That will depend on your applications and how you want your system to function but basically there are 2 choices:
Small clusters make more use of your hard disc space with slower access times.
Large clusters utilise less of your hard disc space but offer a faster access time.
These 2 points are affected by the size of files on your drive (large or small) Example: If you have a 32k cluster size then a 33k file will take up 2 whole clusters - so a hard disc full of small files with a large cluster size will really cut down your hard disc space.
In general large files are better on large cluster sizes and vica versa. That is why many people have their OS on 1 drive and their games etc on another (eg many small .dlls in windows and many large texture / sound files in games)
4.77MHz to 4.0GHz in 10 years. Imagine the space year 2020
Go over to <A HREF="http://www.xbitlabs.com" target="_new">http://www.xbitlabs.com</A> where they have lots of articles on RAID setups tested with different block sizes. They also have their own benchmarking program FC-test which uses different file partterns (OS, MP3s, ISOs etc) to test the drive.
Assuming you got SP1, you could run 'analyze' on XP defragmenter and look at the average file size for the volume. You should choose one that suits this value for optimum performance. There are tools for analyzing a volume for the best cluster size, use a net search engine.
It depends on the usage but speed, space efficency, and reliabilty should be considered as a whole where possible.
Also consider that fault tolerance can be a decision maker when making cluster sizes as above and it's not commonly associated with cluster size, not in my opinion. In general, larger cluster sizes make for more fault tolerant NTFS volumes.