The reality is that you'll see no improvement at all from running ddr2 1333.
You can find p45 mobos that will support up to 16gb of memory, but again unless you're rendering or using your pc as a server 4gb is more than enough for 95pct of things.
If you really want to give your machine a kick, look into an SSD as a system disk. Intel recently released a 40gb version of their popular X25 series. It'll set you back 130 bucks, but there is no other upgrade path that will change the way you look at your pc the way a ssd will.
I have 4GB of DDR3 which could run at 1333Mhz and I know my E8400 can handle 1333 but I'm sure my board doesn't allow for it so I'm looking to fix that but while fixing it I thought it would be nice to do more to get the best out of what I've got. That's why I'm asking about more RAM.
Agreed...unless you have the E8400 clocked up to 3.6Ghz and above, 800 would be a 1:1 ratio. If you go above that, faster RAM may help you out, but as state by a4mula - it seems to play a bigger part in synthetics, etc