You could create a ramdisk and RAID-1 it with a physical partition. Look at the --write-mostly and --write-behind options. You can use those to make the physical disk one which is not to be read from (only written to), and to set the number of outstanding write operations, respectively.
Alternatively, look at the documentation for pdflush. There's a good page here: http://www.westnet.com/~gsmith/content/linux-pdflush.htm (also linked by ire_and_curses)
Beyond what ire mentioned, you'll probably want to crank swappiness up to 100 to favor disk cache over swap.
But it'd be worthwhile to learn how it all works, and tune it to your specific app. Linux is already tuned for the general case, and only you know how your specific situation differs. :)
