An answer (see below) to one of the questions right here on Stack Overflow gave me an idea for a great little piece of software that could be in
In Linux (you never mentioned which OS you're on, so this could be relevant) you can create block devices from RAM and mount them like any other block device (that is, a HDD).
You can then create scripts that copy to and from that drive on start-up / shutdown, as well as periodically.
For example, you could set it up so you had ~/code and ~/code-real. Your RAM block gets mounted at ~/code on startup, and then everything from ~/code-real (which is on your standard hard drive) gets copied over. On shutdown everything would be copied (rsync'd would be faster) back from ~/code to ~/code-real. You would also probably want that script to run periodically, so you didn't lose much work in the event of a power failure, etc.
I don't do this anymore (I used it for Opera when the 9.5 beta was slow, no need anymore).
Here is how to create a RAM disk in Linux.