Upgrade dell dimension 8400

Each bios is different. Probably a 6xxx series, but some might work with the 9xxx. You should post this in the dell forums to see if someone else has tried it. Dell won't help you to upgrade; they want you to buy a new system. If you want to try changing out the motherboard, get a foxconn g41 pr p45 chipset motherboard, and you might be able to avoid a format and use a core 2 quad with your old ram.