firstly, is it the bios not recognising all your ram? if that is the case, then your bios is not configured properly. if your bios is configured properly then your ram is incompatible with you other hardware.
if your ram is specced at 800 mhz and your cpu is capable of supporting 1033mhz that will also slow you down big time and cause these issues.
secondly are you running 64 bit or 32 bit? 32 bit will recognize that you have a lot of ram but will only access 3gigs of it and leave the rest. 64 bit will utilize all of it.
did you reconfigure your bios when you put the new cpu in? if you didn't then just access your bios and then save and exit, that'll set the default settings for the cpu. anytime i add new hardware to my machine whether it's a lan card, more ram, cpu etc i always access the bios then save and exit.
it also sounds like your cpu could be overheating, or your ram, or your puwer supply unit. is your heatsink clean? did you use conductive gel between cpu and heatsink? is your ram clean. is your power supply free from dust?
these are all problems that i've had in the past causing exactly waht you're experiencing now and that is what i did to fix it.