Well, chances are that it's not taking enough of your ram to make much of a difference, but the 8400 would be worth switching to anyway. Can you list your computer specs?
intel celeron d 3.33ghz, 2gb of ddr1 Ram, 160gb harddrive, ACPI uniprocessor
thats all i can get from it, parents got this computer as a family computer and it kinda became mine not that long ago
mayb you can find out more for it, its a eMachines w3503