You have no choice but to buy a new motherboard and new RAM. Upgrading a Dell may very difficult. I've heard that the connecter for the power supply and the socket on the motherboard are specialy designed that only they can fit together, exclusive to Dell. Your old PSU may be too weak anyway, you should have 350Watts+. Personnaly I would buy a whole new computer when you factor in the price of the CPU, RAM, Case/PSU + the hassel. I would get an AMD CPU because they better value for the money.