You should really consider some new upgrade, cause the hardware you have is from the stone age. I also have a core 2 duo (E6400) and i'm about to do some great upgrades.
Its not from the stone age, if it otherwise does the job he needs it to do then he has no problem with the hardware from that point of view.
Hexit has some good points.
Given it only has two ram slots its probably a low end mobo, it may not support 4GB per stick properly, surprised it gets into windows if thats the case.
Doe you have a Physical address extension in the bios, it should be on.
Did you see 4Gb with your old memory, or only 3.25 both in bios and windows.