Yes the hardrive will fragment, but not nearly as much as windows will. There are defragmenter tools for linux, but you should not need them. I have been using RH 9 for about 8 months now and the last time I saw anything on my system was like 3% fragmented. This has not done anything to my systems stability or speed. The linux filesystem takes care of your system pretty well. If you want to take a look at defrag tools, try this: http://www.google.com/linux?hl=en&lr=lang_en&ie=ISO-8859-1&q=defrag&btnG=Google+Search
Hope this helps and if anyone else can elaborate on this, all the better.
In just about all Linux filesystems, writes are held momentarily for other data to be added to it. I forget what the actual term is, but basically, there's a buffer that contains the file to be written to disk. It waits a little while for any more data to be written with it, and then it writes all of the data to disk in one go. Basically, Linux prevents fragmentation by already writing a file to disk all at once, so it's already defragmented on the disk.