The 805 is easily the most overclockable Pentium D available. I really don't see a 945 as any kind of upgrade, myself. I honestly can't see where you would even get any performance gains by making that swap.
Get an E4300 or E6320, a decent 965P based mobo, and 2GB of PC2-6400 and you can make your upgrade for a very reasonable amount of money, and have EXCELLENT performance gains to show for the $$$ spent.