What is the fastest CPU I can upgrade to with a 400 MHz s478 FSB?

Total waste. You do not even have the possibility of dual core processing and it is going to cost you like $80 plus shipping to make the swap. You will gain like 1.0 Mhz and you will not notice a difference. You should start from scratch. If you want to do it cheap I will give you some advice. Build AMD. The technology is slightly behind Intel but the price is literally half. You can get into a dual core system for like $300 if you reuse your video card and power supply. If you want help building a machine email me.