Will this RAM fit my computer? don't want to buy wrong one?

The Dell Dimension 2400 only takes PC-2100 and PC-2700 RAM. You can put in PC-3200 RAM, but it will only run at PC-2700 speeds, so you would have wasted your money. I would recommend PC-2700 RAM, and a 3.06 gigahertz Pentium 4 CPU.