Anything To Get the Most Out of My System

Hello all. I am saving for a new PC, but with my budget I cannot get it until the end of this year. In the meantime, I have a Dell Optiplex 620 with a Pentium D, 3.5 GB of RAM, and an 8800GT 512mb. Until I can afford my new system, what can I do to get the most out of this system? I am open to overclocking, just about anything.
Thanks