Ram change question

First off DDRII will only be a must on i925X upcoming chipset the i915 will let you run DDR and DDRII.
At first DDRII wont give you any perfomance boost at least not clock for clock with DDR because it has higher latency. Even the faster DDRII-533 wont be usefull since P4's are only at at 800mhz(if you use dual channel of 533 you get 1066 so the extra 266mhz wont be of any use unless you overclock your cpu, you can always run memory asynchronously at 5:4 ratio but again it wont really increase your performance...)

Question why you want to sell your DDR500 ram? Whats CPu you got? it might be usefull do squeeze more perfomance out of your current rig. If you have a P4c your probly much better with the extra mhz than lower timmings.As an example my P4 is much faster at 3.25ghz with DDR500 cas3 than it was at 2.6 with DDR 400 cas2 memory.

Asus P4P800DX, P4C 2.6ghz@3.25ghz, 2X512 OCZ PC4000 3-3-3-8, Leadtek FX5900@450/875, 2X30gig Raid0